avcodec/mjpegdec: Check index in ljpeg_decode_yuv_scan() before using it

Fixes: 04715144ba237443010554be0d05343f/asan_heap-oob_1eafc76_1737_c685b48041a563461839e4e7ab97abb8.jpg
Fixes out of array access

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>