avcodec/vc1dec: Clear mb_type_base and ttblk_base

Fixes: two use-of-uninitialized-value
Fixes: 70856/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_VC1IMAGE_fuzzer-5539349918187520

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/vc1dec.c

@@ -379,7 +379,7 @@ static av_cold int vc1_decode_init_alloc_tables(VC1Context *v)
     if (!v->block || !v->cbp_base)
         return AVERROR(ENOMEM);
     v->cbp              = v->cbp_base + 2 * s->mb_stride;
-    v->ttblk_base       = av_malloc(sizeof(v->ttblk_base[0]) * 3 * s->mb_stride);
+    v->ttblk_base       = av_mallocz(sizeof(v->ttblk_base[0]) * 3 * s->mb_stride);
     if (!v->ttblk_base)
         return AVERROR(ENOMEM);
     v->ttblk            = v->ttblk_base + 2 * s->mb_stride;
@@ -393,7 +393,7 @@ static av_cold int vc1_decode_init_alloc_tables(VC1Context *v)
     v->luma_mv          = v->luma_mv_base + 2 * s->mb_stride;
 
     /* allocate block type info in that way so it could be used with s->block_index[] */
-    v->mb_type_base = av_malloc(s->b8_stride * (mb_height * 2 + 1) + s->mb_stride * (mb_height + 1) * 2);
+    v->mb_type_base = av_mallocz(s->b8_stride * (mb_height * 2 + 1) + s->mb_stride * (mb_height + 1) * 2);
     if (!v->mb_type_base)
         return AVERROR(ENOMEM);
     v->mb_type[0]   = v->mb_type_base + s->b8_stride + 1;