Stackless issue #254: fix WITH_CLEANUP_START

Fix the problem introduced by the previous merge of 520b7ae
(bpo-17611).

Author: Anselm Kruis

Python/ceval.c

@@ -3257,7 +3257,7 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
                 slp_continue_slp_eval_frame_with_cleanup:
                 /* Initialize variables */
                 exc = TOP();
-                if (PyLong_Check(exc))
+                if (exc == NULL)
                     exc = Py_None;
                 res = retval;
                 retval = NULL;