Problem: unused result of call to pull_msg

Solution: add TODO comment regarding handling of the result

src/udp_engine.cpp

@@ -361,6 +361,8 @@ void zmq::udp_engine_t::out_event ()
     if (rc == 0) {
         msg_t body_msg;
         rc = _session->pull_msg (&body_msg);
+        //  TODO rc is not checked here. We seem to assume rc == 0. An
+        //  assertion should be added.
 
         const size_t group_size = group_msg.size ();
         const size_t body_size = body_msg.size ();