Fix ast_error_finish() and err_input(): filename can be NULL

Fix my previous commit (r85569).

Python/ast.c

@@ -131,7 +131,12 @@ ast_error_finish(const char *filename)
         Py_INCREF(Py_None);
         loc = Py_None;
     }
-    filename_obj = PyUnicode_DecodeFSDefault(filename);
+    if (filename != NULL)
+        filename_obj = PyUnicode_DecodeFSDefault(filename);
+    else {
+        Py_INCREF(Py_None);
+        filename_obj = Py_None;
+    }
     if (filename_obj != NULL)
         tmp = Py_BuildValue("(NlOO)", filename_obj, lineno, offset, loc);
     else

Python/pythonrun.c

@@ -2054,7 +2054,12 @@ err_input(perrdetail *err)
         errtext = PyUnicode_DecodeUTF8(err->text, strlen(err->text),
                                        "replace");
     }
-    filename = PyUnicode_DecodeFSDefault(err->filename);
+    if (err->filename != NULL)
+        filename = PyUnicode_DecodeFSDefault(err->filename);
+    else {
+        Py_INCREF(Py_None);
+        filename = Py_None;
+    }
     if (filename != NULL)
         v = Py_BuildValue("(NiiN)", filename,
                           err->lineno, err->offset, errtext);