Fixes CVE-2011-3940 (Out of bounds read resulting in out of bounds write)

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 5c011706)
Signed-off-by: Alex Converse <alex.converse@gmail.com>