asf: reset side data elements on packet copy.

Prevents crash (double free) when free()ing the original packet.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavformat/asfdec.c

@@ -1072,6 +1072,8 @@ static int ff_asf_parse_packet(AVFormatContext *s, AVIOContext *pb, AVPacket *pk
             //printf("packet %d %d\n", asf_st->pkt.size, asf->packet_frag_size);
             asf_st->pkt.size = 0;
             asf_st->pkt.data = 0;
+            asf_st->pkt.side_data_elems = 0;
+            asf_st->pkt.side_data = NULL;
             break; // packet completed
         }
     }