Issue #17810: return -1 on error

Modules/_pickle.c

@@ -2941,7 +2941,7 @@ save_frozenset(PicklerObject *self, PyObject *obj)
 
     iter = PyObject_GetIter(obj);
     if (iter == NULL) {
-        return NULL;
+        return -1;
     }
     for (;;) {
         PyObject *item;