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>

libavcodec/vc1dec.c

@@ -5516,7 +5516,7 @@ av_cold int ff_vc1_decode_init_alloc_tables(VC1Context *v)
     v->ttblk            = v->ttblk_base + s->mb_stride;
     v->is_intra_base    = av_mallocz(sizeof(v->is_intra_base[0]) * 2 * s->mb_stride);
     v->is_intra         = v->is_intra_base + s->mb_stride;
-    v->luma_mv_base     = av_malloc(sizeof(v->luma_mv_base[0]) * 2 * s->mb_stride);
+    v->luma_mv_base     = av_mallocz(sizeof(v->luma_mv_base[0]) * 2 * s->mb_stride);
     v->luma_mv          = v->luma_mv_base + s->mb_stride;
 
     /* allocate block type info in that way so it could be used with s->block_index[] */