If someone requests a format at fmt->index == (unsigned)-1 and the first
format in the array doesn't have the requested type then num will still be
-1 when it's compared to fmt->index and there will appear to be a match.

Restructure the loop so this can't happen.  It's simpler this way too.  The
unnecessary check for (unsigned)fmt->index < 0 found by Roel Kluin
<roel.kluin@gmail.com> is removed this way too.
Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>