avformat/takdec: zero padding area before use

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7fcecee73d71_6470_luckynight-partial.tak
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
