threads: fix old frames returned after avcodec_flush_buffers()

Calling avcodec_flush_buffers() and then avcodec_decode_video2() with
a 0-sized packet (to get remaining buffered frames) could incorrectly
return an old frame from before the avcodec_flush_buffers() call. Add
a loop in ff_thread_flush() to zero the got_frame field of each thread
to ensure the old frames will not be returned.

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavcodec/pthread.c

@@ -884,6 +884,9 @@ void ff_thread_flush(AVCodecContext *avctx)
     fctx->next_decoding = fctx->next_finished = 0;
     fctx->delaying = 1;
     fctx->prev_thread = NULL;
+    // Make sure decode flush calls with size=0 won't return old frames
+    for (int i = 0; i < avctx->thread_count; i++)
+        fctx->threads[i].got_frame = 0;
 }
 
 static int *allocate_progress(PerThreadContext *p)