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

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

 /**
  * VP8 compatible video decoder
  *
  * Copyright (C) 2010 David Conrad
  * Copyright (C) 2010 Ronald S. Bultje
  * Copyright (C) 2010 Jason Garrett-Glaser
  *
  * This file is part of FFmpeg.
  *
  * FFmpeg is free software; you can redistribute it and/or
@@ skipping 32 matching lines @@
     av_freep(&s->top_border);
 
     s->macroblocks        = NULL;
 }
 
 static int vp8_alloc_frame(VP8Context *s, AVFrame *f)
 {
     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);
         return AVERROR(ENOMEM);
     }
     return 0;
 }
 
 static void vp8_release_frame(VP8Context *s, AVFrame *f, int prefer_delayed_free, int can_direct_free)
 {
@@ skipping 1493 matching lines @@
         av_freep(&s->segmentation_maps[--s->num_maps_to_be_freed]);
     s->maps_are_invalid = 0;
 }    
 
 static int vp8_decode_frame(AVCodecContext *avctx, void *data, int *data_size,
                             AVPacket *avpkt)
 {
     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;
 
     skip_thresh = !referenced ? AVDISCARD_NONREF :
                     !s->keyframe ? AVDISCARD_NONKEY : AVDISCARD_ALL;
 
     if (avctx->skip_frame >= skip_thresh) {
         s->invisible = 1;
         goto skip_decode;
     }
@@ skipping 251 matching lines @@
     vp8_decode_init,
     NULL,
     vp8_decode_free,
     vp8_decode_frame,
     CODEC_CAP_DR1 | CODEC_CAP_FRAME_THREADS,
     .flush = vp8_decode_flush,
     .long_name = NULL_IF_CONFIG_SMALL("On2 VP8"),
     .init_thread_copy      = ONLY_IF_THREADS_ENABLED(vp8_decode_init_thread_copy),
     .update_thread_context = ONLY_IF_THREADS_ENABLED(vp8_decode_update_thread_context),
 };