avcodec/vc1dec: use av_mallocz for luma_mv

This ensures that no mvs are uninitialized at the time of loop filtering
Fixes: msan_uninit-mem_7f0b6dfe293c_2786_SA20021.vc1
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
