pthread: unref the decoded but not returned frames on close.

Fixes memleaks when frame mt is used and the decoder is not flushed at
the end.

libavcodec/pthread.c

@@ -749,6 +749,7 @@ static void frame_thread_free(AVCodecContext *avctx, int thread_count)
         avctx->codec = NULL;
 
         release_delayed_buffers(p);
+        av_frame_unref(&p->frame);
     }
 
     for (i = 0; i < thread_count; i++) {