Small dangling pointer tweak/fix to CCEventDispatcher

If cleaning a target node that has a listener in the 'to be added' listeners list then ensure that listener does not have a stale reference to the node. This is a minor tweak in addition to pull request 6160 (https://github.com/cocos2d/cocos2d-x/pull/6160).

cocos/2d/CCEventDispatcher.cpp

@@ -351,6 +351,7 @@ void EventDispatcher::removeEventListenersForTarget(Node* target, bool recursive
             
         if (listener->getSceneGraphPriority() == target)
         {
+        	listener->setSceneGraphPriority(nullptr);   // Ensure no dangling ptr to the target node.
             listener->setRegistered(false);
             listener->release();
             iter = _toAddedListeners.erase(iter);