avcodec/mjpegdec: pass into ff_mjpeg_decode_sos() and check bitmask size

Fixes: heap array overread
Fixes: asan_heap-oob_149b2bc_6577_m1.mxg
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
