It can be 0 or -1 for invalid files, which may result in invalid memory
access.
Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Fixes part of Ticket1373
Found-by: Piotr Bandurski <ami_stuff@o2.pl>
Based-on-patch-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>