vc1dec: check end_mb_y / start_mb_y validity

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/vc1dec.c

@@ -5606,6 +5606,10 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
                 s->end_mb_y = (i == n_slices     ) ? mb_height : FFMIN(mb_height, slices[i].mby_start % mb_height);
             else
                 s->end_mb_y = (i <= n_slices1 + 1) ? mb_height : FFMIN(mb_height, slices[i].mby_start % mb_height);
+            if (s->end_mb_y <= s->start_mb_y) {
+                av_log(v->s.avctx, AV_LOG_ERROR, "end mb y %d %d invalid\n", s->end_mb_y, s->start_mb_y);
+                continue;
+            }
             vc1_decode_blocks(v);
             if (i != n_slices)
                 s->gb = slices[i].gb;