avcodec/ivi_common: use av_mallocz() to allocate mbs array

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f1dffa1c016_8245_sasha.avi
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
