vc1dec: Don't decode slices when the latest slice header failed to decode

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/vc1dec.c

@@ -5972,6 +5972,8 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
         if (avctx->hwaccel->end_frame(avctx) < 0)
             goto err;
     } else {
+        int header_ret = 0;
+
         ff_mpeg_er_frame_start(s);
 
         v->bits = buf_size * 8;
@@ -6009,18 +6011,20 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
             if (i) {
                 v->pic_header_flag = 0;
                 if (v->field_mode && i == n_slices1 + 2) {
-                    if (ff_vc1_parse_frame_header_adv(v, &s->gb) < 0) {
+                    if ((header_ret = ff_vc1_parse_frame_header_adv(v, &s->gb)) < 0) {
                         av_log(v->s.avctx, AV_LOG_ERROR, "Field header damaged\n");
                         continue;
                     }
                 } else if (get_bits1(&s->gb)) {
                     v->pic_header_flag = 1;
-                    if (ff_vc1_parse_frame_header_adv(v, &s->gb) < 0) {
+                    if ((header_ret = ff_vc1_parse_frame_header_adv(v, &s->gb)) < 0) {
                         av_log(v->s.avctx, AV_LOG_ERROR, "Slice header damaged\n");
                         continue;
                     }
                 }
             }
+            if (header_ret < 0)
+                continue;
             s->start_mb_y = (i == 0) ? 0 : FFMAX(0, slices[i-1].mby_start % mb_height);
             if (!v->field_mode || v->second_field)
                 s->end_mb_y = (i == n_slices     ) ? mb_height : FFMIN(mb_height, slices[i].mby_start % mb_height);