simple fix (#35)

broker/broker.go

@@ -617,7 +617,9 @@ func (b *Broker) removeClient(c *client) {
 func (b *Broker) PublishMessage(packet *packets.PublishPacket) {
 	var subs []interface{}
 	var qoss []byte
+	b.mu.Lock()
 	err := b.topicsMgr.Subscribers([]byte(packet.TopicName), packet.Qos, &subs, &qoss)
+	b.mu.Unlock()
 	if err != nil {
 		log.Error("search sub client error,  ", zap.Error(err))
 		return

broker/client.go

@@ -213,7 +213,9 @@ func (c *client) ProcessPublishMessage(packet *packets.PublishPacket) {
 		}
 	}
 
+	c.mu.Lock()
 	err := c.topicsMgr.Subscribers([]byte(packet.TopicName), packet.Qos, &c.subs, &c.qoss)
+	c.mu.Unlock()
 	if err != nil {
 		log.Error("Error retrieving subscribers list: ", zap.String("ClientID", c.info.clientID))
 		return