fix leak

pkg/mqtt/connection.go

@@ -306,6 +306,9 @@ func (c *Connection) RcvMsgFromClient() {
 }
 
 func (c *Connection) SendMsgToClient() {
+	if c.Conn == nil {
+		return
+	}
 	host := c.Conn.RemoteAddr()
 	for {
 		c.lock.Lock()