update registeredChannels cache on rename as well

irc/channelmanager.go

 	info = channel.ExportRegistration(IncludeInitial)
 	delete(cm.chans, cfname)
 	cm.chans[cfnewname] = entry
+	if cm.registeredChannels[cfname] {
+		delete(cm.registeredChannels, cfname)
+		cm.registeredChannels[cfnewname] = true
+	}
 	entry.channel.Rename(newname, cfnewname)
 	return nil
 }