Fix invalid reads detected by Valgrind when the frame size changes.

third_party/ffmpeg/source/patched-ffmpeg/libavcodec/vp8.c

Index: third_party/ffmpeg/source/patched-ffmpeg/libavcodec/vp8.c
===================================================================
--- third_party/ffmpeg/source/patched-ffmpeg/libavcodec/vp8.c	(revision 109270)
+++ third_party/ffmpeg/source/patched-ffmpeg/libavcodec/vp8.c	(working copy)
@@ -50,8 +50,7 @@
     int ret;
     if ((ret = ff_thread_get_buffer(s->avctx, f)) < 0)
         return ret;
-    if (s->num_maps_to_be_freed) {
-        assert(!s->maps_are_invalid);
+    if (s->num_maps_to_be_freed && !s->maps_are_invalid) {
         f->ref_index[0] = s->segmentation_maps[--s->num_maps_to_be_freed];
     } else if (!(f->ref_index[0] = av_mallocz(s->mb_width * s->mb_height))) {
         ff_thread_release_buffer(s->avctx, f);
@@ -1565,13 +1564,15 @@
     VP8Context *s = avctx->priv_data;
     int ret, mb_x, mb_y, i, y, referenced;
     enum AVDiscard skip_thresh;
-    AVFrame *av_uninit(curframe), *prev_frame = s->framep[VP56_FRAME_CURRENT];
+    AVFrame *av_uninit(curframe), *prev_frame;
 
     release_queued_segmaps(s, 0);
 
     if ((ret = decode_frame_header(s, avpkt->data, avpkt->size)) < 0)
         return ret;
 
+    prev_frame = s->framep[VP56_FRAME_CURRENT];
+
     referenced = s->update_last || s->update_golden == VP56_FRAME_CURRENT
                                 || s->update_altref == VP56_FRAME_CURRENT;