Fix crash in exec when unicode filename can't be decoded.

I can't think of an easy way to test this behavior.  It only occurs
when the file system default encoding and the interpreter default
encoding are different, such that you can open the file but not decode
its name.

Misc/NEWS

@@ -121,6 +121,7 @@ Core and builtins
 
 - with and as are now keywords.
 
+- Bug #1664966: Fix crash in exec if Unicode filename can't be decoded.
 
 Library
 -------

Python/ceval.c

@@ -4202,6 +4202,8 @@ exec_statement(PyFrameObject *f, PyObject *prog, PyObject *globals,
 	else if (PyFile_Check(prog)) {
 		FILE *fp = PyFile_AsFile(prog);
 		char *name = PyString_AsString(PyFile_Name(prog));
+                if (name == NULL)
+                        return -1;
 		PyCompilerFlags cf;
 		cf.cf_flags = 0;
 		if (PyEval_MergeCompilerFlags(&cf))