Don't crash when nonlocal is used at module level (fixes SF#1705365)

Lib/test/test_syntax.py

@@ -388,6 +388,12 @@ Misuse of the nonlocal statement can lead to a few unique syntax errors.
      ...
    SyntaxError: no binding for nonlocal 'x' found
 
+From SF bug #1705365
+   >>> nonlocal x
+   Traceback (most recent call last):
+     ...
+   SyntaxError: nonlocal declaration not allowed at module level
+
 TODO(jhylton): Figure out how to test SyntaxWarning with doctest.
 
 ##   >>> def f(x):

Python/symtable.c

@@ -337,8 +337,6 @@ PyST_GetScope(PySTEntryObject *ste, PyObject *name)
    block, the name is treated as global until it is assigned to; then it
    is treated as a local.
 
-   TODO(jhylton): Discuss nonlocal
-
    The symbol table requires two passes to determine the scope of each name.
    The first pass collects raw facts from the AST via the symtable_visit_*
    functions: the name is a parameter here, the name is used but not defined
@@ -348,8 +346,10 @@ PyST_GetScope(PySTEntryObject *ste, PyObject *name)
    When a function is entered during the second pass, the parent passes
    the set of all name bindings visible to its children.  These bindings 
    are used to determine if non-local variables are free or implicit globals.
-   After doing the local analysis, it analyzes each of its child blocks
-   using an updated set of name bindings.
+   Names which are explicitly declared nonlocal must exist in this set of
+   visible names - if they do not, a syntax error is raised. After doing
+   the local analysis, it analyzes each of its child blocks using an
+   updated set of name bindings.
 
    The children update the free variable set.  If a local variable is added to
    the free variable set by the child, the variable is marked as a cell.  The
@@ -415,6 +415,11 @@ analyze_name(PySTEntryObject *ste, PyObject *scopes, PyObject *name, long flags,
 				     PyString_AS_STRING(name));
 			return 0;
 		}
+		if (!bound) {
+			PyErr_Format(PyExc_SyntaxError,
+				     "nonlocal declaration not allowed at module level");
+                        return 0;
+		}
                 if (!PySet_Contains(bound, name)) {
                         PyErr_Format(PyExc_SyntaxError,
                                      "no binding for nonlocal '%s' found",