Fix the C implementation

Author: vstinner

Modules/_pickle.c

@@ -3666,6 +3666,19 @@ save_frozendict(PickleState *state, PicklerObject *self, PyObject *obj)
         }
     }
 
+    /* If the object is already in the memo, this means it is
+       recursive. In this case, throw away everything we put on the
+       stack, and fetch the object back from the memo. */
+    if (PyMemoTable_Get(self->memo, obj)) {
+        const char pop_mark_op = POP_MARK;
+
+        if (_Pickler_Write(self, &pop_mark_op, 1) < 0)
+            return -1;
+        if (memo_get(state, self, obj) < 0)
+            return -1;
+        return 0;
+    }
+
     const char frozendict_op = FROZENDICT;
     if (_Pickler_Write(self, &frozendict_op, 1) < 0) {
         return -1;