avcodec/mjpegdec: use mallocz for progressive blocks

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f849c599487_6828_mjpeg.avi
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
