bpo-32436: Fix compiler warning (GH-5483) (GH-5486)

(cherry picked from commit 55e0839)

Co-authored-by: Yury Selivanov <yury@magic.io>

Author: miss-islington

Python/hamt.c

@@ -2358,6 +2358,8 @@ _PyHamt_Without(PyHamtObject *o, PyObject *key)
             Py_INCREF(o);
             return o;
         case W_NEWNODE: {
+            assert(new_root != NULL);
+
             PyHamtObject *new_o = hamt_alloc();
             if (new_o == NULL) {
                 Py_DECREF(new_root);