les: fix server panic when discovery disabled (#16055)

les/server.go

@@ -111,6 +111,7 @@ func (s *LesServer) Protocols() []p2p.Protocol {
 // Start starts the LES server
 func (s *LesServer) Start(srvr *p2p.Server) {
 	s.protocolManager.Start(s.config.LightPeers)
+	if srvr.DiscV5 != nil {
 		for _, topic := range s.lesTopics {
 			topic := topic
 			go func() {
@@ -121,6 +122,7 @@ func (s *LesServer) Start(srvr *p2p.Server) {
 				srvr.DiscV5.RegisterTopic(topic, s.quitSync)
 			}()
 		}
+	}
 	s.privateKey = srvr.PrivateKey
 	s.protocolManager.blockLoop()
 }