fix: conn nil

pkg/mqtt/connection.go

@@ -315,17 +315,17 @@ func (c *Connection) RcvMsgFromClient() {
 }
 
 func (c *Connection) SendMsgToClient() {
-	c.mLock.Lock()
-	c.mLock.Unlock()
 	host := c.Conn.RemoteAddr()
 	for {
 		select {
 		case <-c.closeChan:
 			return
 		case msg, ok := <-c.SendChan:
+			c.mLock.Lock()
 			if c.Conn == nil {
 				return
 			}
+			c.mLock.Unlock()
 			if !ok {
 				server.Log.Errorf("%s is end now", host)
 				return