Fix segfault reported with ftpalias.cfg support in some cases
ftpalias() can return true even when the directory is not set to a valid directory index (i.e. set to -1), so using as an array index would definitely segfault. Part of commit 8ad30b6c by Deuce 3 years ago. I didn't test this as I'm not sure exactly the combination of ftpalias.cfg content and FTP command received that would trigger this, but it's most definitely a bug. So should fix the segfault reported in issue #288.