when topic destroying. make nsqd won't assign any new client into it

fix #883

nsqd/nsqd.go

@@ -501,13 +501,12 @@ func (n *NSQD) GetExistingTopic(topicName string) (*Topic, error) {
 
 // DeleteExistingTopic removes a topic only if it exists
 func (n *NSQD) DeleteExistingTopic(topicName string) error {
-	n.RLock()
+	n.Lock()
 	topic, ok := n.topicMap[topicName]
 	if !ok {
-		n.RUnlock()
+		n.Unlock()
 		return errors.New("topic does not exist")
 	}
-	n.RUnlock()
 
 	// delete empties all channels and the topic itself before closing
 	// (so that we dont leave any messages around)
@@ -517,7 +516,6 @@ func (n *NSQD) DeleteExistingTopic(topicName string) error {
 	// to enforce ordering
 	topic.Delete()
 
-	n.Lock()
 	delete(n.topicMap, topicName)
 	n.Unlock()