avcodec/mjpegdec: use mallocz for progressive blocks

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f849c599487_6828_mjpeg.avi
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mjpegdec.c

@@ -510,7 +510,7 @@ unk_pixfmt:
             int size = bw * bh * s->h_count[i] * s->v_count[i];
             av_freep(&s->blocks[i]);
             av_freep(&s->last_nnz[i]);
-            s->blocks[i]       = av_malloc(size * sizeof(**s->blocks));
+            s->blocks[i]       = av_mallocz(size * sizeof(**s->blocks));
             s->last_nnz[i]     = av_mallocz(size * sizeof(**s->last_nnz));
             if (!s->blocks[i] || !s->last_nnz[i])
                 return AVERROR(ENOMEM);