pthread_frame: flush all threads on flush, not just the first one

avcodec_flush_buffers() must release all internally held references
according to its documentation, for which all the threads need to be
flushed.

CC:libav-stable@libav.org
Bug-Id: vlc/9665

libavcodec/pthread_frame.c

@@ -672,8 +672,6 @@ void ff_thread_flush(AVCodecContext *avctx)
     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;
@@ -686,6 +684,9 @@ void ff_thread_flush(AVCodecContext *avctx)
         av_frame_unref(p->frame);
 
         release_delayed_buffers(p);
+
+        if (avctx->codec->flush)
+            avctx->codec->flush(p->avctx);
     }
 }