udp: handle closed forwarding channel (#5470)

pkg/proto/udp/udp.go

@@ -63,9 +63,13 @@ func ForwardUserConn(udpConn *net.UDPConn, readCh <-chan *msg.UDPPacket, sendCh
 		// NewUDPPacket copies buf[:n], so the read buffer can be reused
 		udpMsg := NewUDPPacket(buf[:n], nil, remoteAddr)
 
-		select {
-		case sendCh <- udpMsg:
-		default:
+		if err = errors.PanicToError(func() {
+			select {
+			case sendCh <- udpMsg:
+			default:
+			}
+		}); err != nil {
+			return
 		}
 	}
 }