state: Fix voice state removal condition

This commit addresses issue #220.

state/state_events.go

@@ -327,7 +327,7 @@ func (s *State) onEvent(iface interface{}) {
 
 	case *gateway.VoiceStateUpdateEvent:
 		vs := &ev.VoiceState
-		if vs.ChannelID == 0 {
+		if !vs.ChannelID.IsValid() {
 			if err := s.Cabinet.VoiceStateRemove(vs.GuildID, vs.UserID); err != nil {
 				s.stateErr(err, "failed to remove voice state from state")
 			}