vp8/mt: flush worker thread, not application thread context, on seek.

This prevents a crash when seeking.

libavcodec/pthread.c

@@ -747,9 +747,12 @@ void ff_thread_flush(AVCodecContext *avctx)
     if (!avctx->thread_opaque) return;
 
     park_frame_worker_threads(fctx, avctx->thread_count);
-
-    if (fctx->prev_thread)
-        update_context_from_thread(fctx->threads->avctx, fctx->prev_thread->avctx, 0);
+    if (fctx->prev_thread) {
+        if (fctx->prev_thread != &fctx->threads[0])
+            update_context_from_thread(fctx->threads[0].avctx, fctx->prev_thread->avctx, 0);
+        if (avctx->codec->flush)
+            avctx->codec->flush(fctx->threads[0].avctx);
+    }
 
     fctx->next_decoding = fctx->next_finished = 0;
     fctx->delaying = 1;

libavcodec/utils.c

@@ -1080,7 +1080,7 @@ void avcodec_flush_buffers(AVCodecContext *avctx)
 {
     if(HAVE_PTHREADS && avctx->active_thread_type&FF_THREAD_FRAME)
         ff_thread_flush(avctx);
-    if(avctx->codec->flush)
+    else if(avctx->codec->flush)
         avctx->codec->flush(avctx);
 }