pthread: unref already decoded frames when flushing the decoder

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/pthread.c

@@ -890,6 +890,7 @@ void ff_thread_flush(AVCodecContext *avctx)
         PerThreadContext *p = &fctx->threads[i];
         // Make sure decode flush calls with size=0 won't return old frames
         p->got_frame = 0;
+        av_frame_unref(&p->frame);
 
         release_delayed_buffers(p);
     }