avcodec/bsf: Unref the packet when flushing

The documentation does not require the packet to be blank in this case
(i.e. it can now contain opaque_ref), but it does contain that the
contents will be reset upon success.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/bsf.c

@@ -201,6 +201,8 @@ int av_bsf_send_packet(AVBSFContext *ctx, AVPacket *pkt)
     int ret;
 
     if (!pkt || IS_EMPTY(pkt)) {
+        if (pkt)
+            av_packet_unref(pkt);
         bsfi->eof = 1;
         return 0;
     }