avcodec/apedec: use av_fast_padded_malloc()

Fixes use of uninitialized memory
Fixes part of msan_uninit-mem_7fa0d8c8bd58_8417_sh3.ape
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
