avcodec/vqavideo: clear uninitialized parts of the decode buffer

Fixes use of uninitialized memory
Fixes part of msan_uninit-mem_7f841fe2ab3b_4608_cow2_1.vqa
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
