avcodec/mjpegdec: Fix integer overflow in shift

Fixes: signal_sigabrt_7ffff6ac7bb9_2683_cov_4120310995_m_ijpg.avi
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
