avcodec/vqavideo: use av_mallocz() for decode_buffer()

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f62eaba897d_595_4D6EFA92.VQA
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>