avcodec/mjpegdec: check len in mjpeg_decode_app() more completely

Avoids len from becoming negative and causing assertion failure

Fixes: signal_sigabrt_7ffff7126425_5140_fd44dc63fa7bdd12ee34fc602231ef02.jpg

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