optimize code (#471) (#484)

Author: stulzq

Diff for: clients/naming_client/push_receiver.go

@@ -71,8 +71,9 @@ func (us *PushReceiver) tryListen() (*net.UDPConn, bool) {
 }
 
 func (us *PushReceiver) getConn() *net.UDPConn {
+	r := rand.New(rand.NewSource(time.Now().UnixNano()))
+
 	for i := 0; i < 3; i++ {
-		r := rand.New(rand.NewSource(time.Now().UnixNano()))
 		port := r.Intn(1000) + 54951
 		us.port = port
 		conn, ok := us.tryListen()