avcodec/binkaudio: clear padding area of packet_buffer

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f0dba198b34_2970_Snd0a3a2ad4.dee
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
