avcodec/vp56: Clear dimensions in case of failure in the middle of a resolution change

Similar code is used elsewhere in vp56 to force a more complete reinit in the future.
Fixes null pointer dereference
Fixes: 707/clusterfuzz-testcase-4717453097566208

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

@@ -572,13 +572,18 @@ int ff_vp56_decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
     }
 
     ret = ff_get_buffer(avctx, p, AV_GET_BUFFER_FLAG_REF);
-    if (ret < 0)
+    if (ret < 0) {
+        if (res == VP56_SIZE_CHANGE)
+            ff_set_dimensions(avctx, 0, 0);
         return ret;
+    }
 
     if (avctx->pix_fmt == AV_PIX_FMT_YUVA420P) {
         av_frame_unref(s->alpha_context->frames[VP56_FRAME_CURRENT]);
         if ((ret = av_frame_ref(s->alpha_context->frames[VP56_FRAME_CURRENT], p)) < 0) {
             av_frame_unref(p);
+            if (res == VP56_SIZE_CHANGE)
+                ff_set_dimensions(avctx, 0, 0);
             return ret;
         }
     }