Avoid queuing null pointers to be freed

Former-commit-id: f50541d437cd1d98139c29407bb3fb4d6ce17fca

src/snapshot.cpp

@@ -403,7 +403,8 @@ void redisDbPersistentData::endSnapshot(const redisDbPersistentDataSnapshot *psn
         }
         
         // Delete the object from the source dict, we don't use dictDelete to avoid a second search
-        splazy->vecde.push_back(deSnapshot);
+        if (deSnapshot != nullptr)
+            splazy->vecde.push_back(deSnapshot);
         *dePrev = deSnapshot->next;
         ht->used--;
     }