lavf/tee: fix side data double free.

Similar to 33fefdb4.
Fix trac ticket #4921.
Signed-off-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/tee.c

@@ -424,6 +424,8 @@ static int filter_packet(void *log_ctx, AVPacket *pkt,
         }
 
         if (ret > 0) {
+            pkt->side_data = NULL;
+            pkt->side_data_elems = 0;
             av_packet_unref(pkt);
             new_pkt.buf = av_buffer_create(new_pkt.data, new_pkt.size,
                                            av_buffer_default_free, NULL, 0);