tsdemux: fix to keep seqnum of segment

Submitted by Wonchul Lee

Description

I tried to seek on hls stream, here is the url : https://mnmedias.api.telequebec.tv/m3u8/29880.m3u8

Hlsdemux handled the seek event and generated relevant flush-start/stop and segment event, but the tsdemux didn't keep the seqnum of segment which receiving event from hlsdemux in this case. So I fixed to keep seqnum of segment in tsdemux for this.