fix udp

Workerman/Worker.php

@@ -1321,11 +1321,15 @@ class Worker
         $connection = new UdpConnection($socket, $remote_address);
         if($this->onMessage)
         {
-            $parser = $this->_protocol;
+            if($this->_protocol)
+            {
+                $parser = $this->_protocol;
+                $recv_buffer = $parser::decode($recv_buffer, $connection);
+            }
             ConnectionInterface::$statistics['total_request']++;
             try
             {
-               call_user_func($this->onMessage, $connection, $parser::decode($recv_buffer, $connection));
+               call_user_func($this->onMessage, $connection, $recv_buffer);
             }
             catch(Exception $e)
             {