avcodec/mjpegdec: Check for pixfmtid 0x42111100 || 0x24111100 with more than 8 bits

These cases are not supported yet

Fixes assertion failure
Fixes: signal_sigabrt_7ffff6ac7bb9_1_cov_1553101927_00.jpg
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>