Coverity complains about a possible sign extension issue in
apply_delogo(). While it is extremely unlikely to happen, it is easy
to fix so let's just do that. Using unsigned variables even makes the
binary code smaller.

Fixes Coverity CID 1046439.
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>