huffyuv: Check and propagate function return values

Bug-Id: CVE-2013-0868

inspired by a patch from Michael Niedermayer <michaelni@gmx.at>
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Diego Biurrun <diego@biurrun.de>

CC: libav-stable@libav.org