Ensure co_stacksize >= 1, else RETURN_CONST may crash

Author: gvanrossum

Objects/codeobject.c

@@ -395,6 +395,9 @@ init_code(PyCodeObject *co, struct _PyCodeConstructor *con)
     int nlocals, ncellvars, nfreevars;
     get_localsplus_counts(con->localsplusnames, con->localspluskinds,
                           &nlocals, &ncellvars, &nfreevars);
+    if (con->stacksize == 0) {
+        con->stacksize = 1;
+    }
 
     co->co_filename = Py_NewRef(con->filename);
     co->co_name = Py_NewRef(con->name);