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

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

 /*
  * Copyright (c) 2004 Roman Shaposhnik
  * Copyright (c) 2008 Alexander Strange (astrange@ithinksw.com)
  *
  * Many thanks to Steven M. Schultz for providing clever ideas and
  * to Michael Niedermayer <michaelni@gmx.at> for writing initial
  * implementation.
  *
  * This file is part of FFmpeg.
  *
@@ skipping 467 matching lines @@
             if (p->state == STATE_GET_BUFFER) {
                 p->result = p->avctx->get_buffer(p->avctx, p->requested_frame);
                 p->state  = STATE_SETTING_UP;
                 pthread_cond_signal(&p->progress_cond);
             }
             pthread_mutex_unlock(&p->progress_mutex);
         }
     }
 
     fctx->prev_thread = p;
+    fctx->next_decoding++;
 
     return 0;
 }
 
 int ff_thread_decode_frame(AVCodecContext *avctx,
                            AVFrame *picture, int *got_picture_ptr,
                            AVPacket *avpkt)
 {
     FrameThreadContext *fctx = avctx->thread_opaque;
     int finished = fctx->next_finished;
     PerThreadContext *p;
     int err;
 
     /*
      * Submit a packet to the next decoding thread.
      */
 
     p = &fctx->threads[fctx->next_decoding];
     update_context_from_user(p->avctx, avctx);
     err = submit_packet(p, avpkt);
     if (err) return err;
 
-    fctx->next_decoding++;
-
     /*
      * If we're still receiving the initial packets, don't return a frame.
      */
 
     if (fctx->delaying && avpkt->size) {
         if (fctx->next_decoding >= (avctx->thread_count-1)) fctx->delaying = 0;
 
         *got_picture_ptr=0;
         return 0;
     }
@@ skipping 388 matching lines @@
     return 0;
 }
 
 void ff_thread_free(AVCodecContext *avctx)
 {
     if (avctx->active_thread_type&FF_THREAD_FRAME)
         frame_thread_free(avctx, avctx->thread_count);
     else
         thread_free(avctx);
 }