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.