avcodec/mjpegdec: Check number of components for JPEG-LS

      Fixes out of array accesses
Fixes: asan_heap-oob_1c1a4ea_1242_cov_2274415971_TESTcmyk.jpg

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit fabbfaa095660982cc0bc63242c459561fa37037)

Conflicts:

	libavcodec/mjpegdec.c