vp56:Fix error recovery code on size changes in vp5/6 decoder

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 1aad9cd9d2)

libavcodec/vp56.c

@@ -465,6 +465,7 @@ static int vp56_size_changed(AVCodecContext *avctx)
     s->mb_height = (avctx->coded_height+15) / 16;
 
     if (s->mb_width > 1000 || s->mb_height > 1000) {
+        avcodec_set_dimensions(avctx, 0, 0);
         av_log(avctx, AV_LOG_ERROR, "picture too big\n");
         return -1;
     }
@@ -519,8 +520,10 @@ int ff_vp56_decode_frame(AVCodecContext *avctx, void *data, int *data_size,
                 if (s->frames[i].data[0])
                     avctx->release_buffer(avctx, &s->frames[i]);
             }
-            if (is_alpha)
+            if (is_alpha) {
+                avcodec_set_dimensions(avctx, 0, 0);
                 return -1;
+            }
         }
 
         if (!is_alpha) {