The reimplementation by Libav does not prevent out of array
writes, even though it looks like it does at a quick glance.

No FFmpeg releases are affected by this

See: d1c95d2c
     3623589e
     740ebe46

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>