We need to track invalid targets as well as 0 targets, so just make it
signed so our comparisons work like they should.
Reviewed-by: Mario Kleiner <mario.kleiner@tuebingen.mpg.de>
Reported-by: Kristian Høgsberg <krh@bitplanet.net>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>