Don't send "Conference" event if user couldn't join room.

go/channelling/api/handle_hello.go

@@ -48,6 +48,10 @@ func (api *channellingAPI) HandleHello(session *channelling.Session, hello *chan
 }
 
 func (api *channellingAPI) HelloProcessed(sender channelling.Sender, session *channelling.Session, msg *channelling.DataIncoming, reply interface{}, err error) {
+	if err != nil {
+		return
+	}
+
 	// If user joined a server-managed conference room, send list of session ids to all participants.
 	if room, ok := api.RoomStatusManager.Get(session.Roomid); ok && room.GetType() == "Conference" {
 		if sessionids := room.SessionIDs(); len(sessionids) > 1 {