avcodec/utils: Align YUV411 by as much as the other YUV variants

Fixes out of array accesses
Fixes: ffmpeg_mjpeg_crash2.avi

Found-by: Thomas Lindroth <thomas.lindroth@gmail.com>
Tested-by: Thomas Lindroth <thomas.lindroth@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
