Check the return values for all functions returning an ast node.

Failure to do it may result in strange error messages or even crashes,
in admittedly convoluted cases that are normally syntax errors, like:
    def f(*xx, __debug__): pass

Python/ast.c

@@ -688,6 +688,8 @@ handle_keywordonly_args(struct compiling *c, const node *n, int start,
             case tfpdef:
                 if (i + 1 < NCH(n) && TYPE(CHILD(n, i + 1)) == EQUAL) {
                     expression = ast_for_expr(c, CHILD(n, i + 2));
+                    if (!expression)
+		      goto error;
                     asdl_seq_SET(kwdefaults, j, expression);
                     i += 2; /* '=' and test */
                 }
@@ -697,10 +699,8 @@ handle_keywordonly_args(struct compiling *c, const node *n, int start,
                 if (NCH(ch) == 3) {
                     /* ch is NAME ':' test */
                     annotation = ast_for_expr(c, CHILD(ch, 2));
-                    if (!annotation) {
-                        ast_error(ch, "expected expression");
+                    if (!annotation)
                         goto error;
-                    }
                 }
                 else {
                     annotation = NULL;
@@ -794,22 +794,22 @@ ast_for_arguments(struct compiling *c, const node *n)
     }
     posargs = (nposargs ? asdl_seq_new(nposargs, c->c_arena) : NULL);
     if (!posargs && nposargs)
-        goto error;
+        return NULL;
     kwonlyargs = (nkwonlyargs ?
                    asdl_seq_new(nkwonlyargs, c->c_arena) : NULL);
     if (!kwonlyargs && nkwonlyargs)
-        goto error;
+        return NULL;
     posdefaults = (nposdefaults ?
                     asdl_seq_new(nposdefaults, c->c_arena) : NULL);
     if (!posdefaults && nposdefaults)
-        goto error;
+        return NULL;
     /* The length of kwonlyargs and kwdefaults are same
        since we set NULL as default for keyword only argument w/o default
        - we have sequence data structure, but no dictionary */
     kwdefaults = (nkwonlyargs ?
                    asdl_seq_new(nkwonlyargs, c->c_arena) : NULL);
     if (!kwdefaults && nkwonlyargs)
-        goto error;
+        return NULL;
 
     if (nposargs + nkwonlyargs > 255) {
         ast_error(n, "more than 255 arguments");
@@ -833,7 +833,7 @@ ast_for_arguments(struct compiling *c, const node *n)
                 if (i + 1 < NCH(n) && TYPE(CHILD(n, i + 1)) == EQUAL) {
                     expr_ty expression = ast_for_expr(c, CHILD(n, i + 2));
                     if (!expression)
-                        goto error;
+                        return NULL;
                     assert(posdefaults != NULL);
                     asdl_seq_SET(posdefaults, j++, expression);
                     i += 2;
@@ -842,11 +842,11 @@ ast_for_arguments(struct compiling *c, const node *n)
                 else if (found_default) {
                     ast_error(n,
                              "non-default argument follows default argument");
-                    goto error;
+                    return NULL;
                 }
                 arg = compiler_arg(c, ch);
                 if (!arg)
-                    goto error;
+                    return NULL;
                 asdl_seq_SET(posargs, k++, arg);
                 i += 2; /* the name and the comma */
                 break;
@@ -854,7 +854,7 @@ ast_for_arguments(struct compiling *c, const node *n)
                 if (i+1 >= NCH(n)) {
                     ast_error(CHILD(n, i),
                         "named arguments must follow bare *");
-                    goto error;
+                    return NULL;
                 }
                 ch = CHILD(n, i+1);  /* tfpdef or COMMA */
                 if (TYPE(ch) == COMMA) {
@@ -862,7 +862,7 @@ ast_for_arguments(struct compiling *c, const node *n)
                     i += 2; /* now follows keyword only arguments */
                     res = handle_keywordonly_args(c, n, i,
                                                   kwonlyargs, kwdefaults);
-                    if (res == -1) goto error;
+                    if (res == -1) return NULL;
                     i = res; /* res has new position to process */
                 }
                 else {
@@ -874,6 +874,8 @@ ast_for_arguments(struct compiling *c, const node *n)
                     if (NCH(ch) > 1) {
                         /* there is an annotation on the vararg */
                         varargannotation = ast_for_expr(c, CHILD(ch, 2));
+                        if (!varargannotation)
+                            return NULL;
                     }
                     i += 3;
                     if (i < NCH(n) && (TYPE(CHILD(n, i)) == tfpdef
@@ -881,7 +883,7 @@ ast_for_arguments(struct compiling *c, const node *n)
                         int res = 0;
                         res = handle_keywordonly_args(c, n, i,
                                                       kwonlyargs, kwdefaults);
-                        if (res == -1) goto error;
+                        if (res == -1) return NULL;
                         i = res; /* res has new position to process */
                     }
                 }
@@ -893,26 +895,24 @@ ast_for_arguments(struct compiling *c, const node *n)
                 if (NCH(ch) > 1) {
                     /* there is an annotation on the kwarg */
                     kwargannotation = ast_for_expr(c, CHILD(ch, 2));
+                    if (!kwargannotation)
+                        return NULL;
                 }
                 if (!kwarg)
-                    goto error;
+                    return NULL;
                 if (forbidden_name(kwarg, CHILD(ch, 0), 0))
-                    goto error;
+                    return NULL;
                 i += 3;
                 break;
             default:
                 PyErr_Format(PyExc_SystemError,
                              "unexpected node in varargslist: %d @ %d",
                              TYPE(ch), i);
-                goto error;
+                return NULL;
         }
     }
     return arguments(posargs, vararg, varargannotation, kwonlyargs, kwarg,
                     kwargannotation, posdefaults, kwdefaults, c->c_arena);
- error:
-    Py_XDECREF(vararg);
-    Py_XDECREF(kwarg);
-    return NULL;
 }
 
 static expr_ty
@@ -997,9 +997,9 @@ ast_for_decorators(struct compiling *c, const node *n)
 
     for (i = 0; i < NCH(n); i++) {
         d = ast_for_decorator(c, CHILD(n, i));
-            if (!d)
-                return NULL;
-            asdl_seq_SET(decorator_seq, i, d);
+        if (!d)
+            return NULL;
+        asdl_seq_SET(decorator_seq, i, d);
     }
     return decorator_seq;
 }
@@ -1027,7 +1027,7 @@ ast_for_funcdef(struct compiling *c, const node *n, asdl_seq *decorator_seq)
     if (TYPE(CHILD(n, name_i+2)) == RARROW) {
         returns = ast_for_expr(c, CHILD(n, name_i + 3));
         if (!returns)
-                return NULL;
+            return NULL;
         name_i += 2;
     }
     body = ast_for_suite(c, CHILD(n, name_i + 3));
@@ -2136,11 +2136,10 @@ ast_for_expr_stmt(struct compiling *c, const node *n)
                 return NULL;
             }
             e = ast_for_testlist(c, ch);
-
-            /* set context to assign */
             if (!e)
               return NULL;
 
+            /* set context to assign */
             if (!set_context(c, e, Store, CHILD(n, i)))
               return NULL;
 
@@ -2960,6 +2959,8 @@ ast_for_with_item(struct compiling *c, const node *n, asdl_seq *content)
 
     REQ(n, with_item);
     context_expr = ast_for_expr(c, CHILD(n, 0));
+    if (!context_expr)
+        return NULL;
     if (NCH(n) == 3) {
         optional_vars = ast_for_expr(c, CHILD(n, 2));