Fixes out of array access
Fixes: case2_bad_read_yuv2rgbx32.mp4
Found-by: Michal Zalewski <lcamtuf@coredump.cx>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 2a983ff7)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>