Upgrading fromm v3.19 can result in a setting of filename_maxlen=0 in file.ini, so let's
put a special check in place to insure this can't happen (thus preventing any file
uploads). I chose '8' as a kind of arbitrary minimum maximum value (MS-DOS filenames
maxed out at 12 characters), but at least 0 won't be possibly any longer.

As reported on DOVE-Net by Codefenix and confirmed by xbit.