Fix listener race condition

[HashimTheArab]

this causes a "send on nil channel" panic rarely
making sure listener.close is closed first will make the select return and drop further processing
then we can safely close listener.incoming

gophertunnel/minecraft/listener.go

Lines 322 to 331 in fdd82ea

	select {
	case <-listener.close:
	// The listener was closed while this one was logged in, so the incoming channel will be
	// closed. Just return so the connection is closed and cleaned up.
	return
	case listener.incoming <- conn:
	// The connection was previously not logged in, but was after receiving this packet,
	// meaning the connection is fully completely now. We add it to the channel so that
	// a call to Accept() can receive it.
	}