avcodec/vp56: Require a correctly decoded frame before using vp56_conceal_mb()

Fixes timeout with 700/clusterfuzz-testcase-5660909504561152
Fixes timeout with 702/clusterfuzz-testcase-4553541576294400

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

libavcodec/vp56.c

@@ -617,8 +617,12 @@ int ff_vp56_decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
         }
     }
 
+    s->discard_frame = 0;
     avctx->execute2(avctx, ff_vp56_decode_mbs, 0, 0, (avctx->pix_fmt == AV_PIX_FMT_YUVA420P) + 1);
 
+    if (s->discard_frame)
+        return AVERROR_INVALIDDATA;
+
     if ((res = av_frame_ref(data, p)) < 0)
         return res;
     *got_frame = 1;
@@ -704,8 +708,13 @@ static int ff_vp56_decode_mbs(AVCodecContext *avctx, void *data,
         for (mb_col=0; mb_col<s->mb_width; mb_col++) {
             if (!damaged) {
                 int ret = vp56_decode_mb(s, mb_row, mb_col, is_alpha);
-                if (ret < 0)
+                if (ret < 0) {
                     damaged = 1;
+                    if (!s->have_undamaged_frame) {
+                        s->discard_frame = 1;
+                        return AVERROR_INVALIDDATA;
+                    }
+                }
             }
             if (damaged)
                 vp56_conceal_mb(s, mb_row, mb_col, is_alpha);
@@ -722,6 +731,9 @@ static int ff_vp56_decode_mbs(AVCodecContext *avctx, void *data,
         }
     }
 
+    if (!damaged)
+        s->have_undamaged_frame = 1;
+
 next:
     if (p->key_frame || s->golden_frame) {
         av_frame_unref(s->frames[VP56_FRAME_GOLDEN]);

libavcodec/vp56.h

@@ -203,6 +203,9 @@ struct vp56_context {
     VLC runv_vlc[2];
     VLC ract_vlc[2][3][6];
     unsigned int nb_null[2][2];       /* number of consecutive NULL DC/AC */
+
+    int have_undamaged_frame;
+    int discard_frame;
 };