Fix sequence return logic in getSeqNumNearestPTS

Simple workaround to fix #23:

* pass `current_seqnum` to `getSeqNumNearestPTS`
* when looping through valid fragments, only return if `fragments[i].seqnum > current_seqnum`

At first glance this seems correct and has fixed the issue we were experiencing, would appreciate your thoughts! cc @caseypugh