avformat/mpc8: clear buffer padding area

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7fa6a48cd1d5_8853_sv8_notags.mpc
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
