avcodec/bink: use av_mallocz for data

Fixes use of uninitialized memory
Fixes msan_uninit-mem_7fe8a5fd759d_2838_SPECTRE.BIK
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
