fix memory leak (cocos2d#18351)

fix memory leak in `UserDefault::deleteValueForKey` when `key` not found

Author: summerinsects

cocos/base/CCUserDefault.cpp

@@ -522,6 +522,7 @@ void UserDefault::deleteValueForKey(const char* key)
     // if node not exist, don't need to delete
     if (!node)
     {
+        CC_SAFE_DELETE(doc);
         return;
     }