avcodec/mjpegdec: Print the number of bits in the unsupported pixel format error

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/mjpegdec: Add YUVA420 formats to *scale asserts

Fixes assertion failure
Fixes: signal_sigabrt_7ffff6ac7bb9_2042_cov_2593130068_ef1f8a057bb6056674fad92f6b8c0acd.jpg
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>