dashdemux: Rename functions that talk about the current segment to say so instead of "next"

Submitted by Florin Apostol

Link to original bug (#752085)

Description

The changes introduced by https://bugzilla.gnome.org/show_bug.cgi?id=751850 are wrong. The segment_index indicates the next segment to be fetched while the gst_mpd_client_advance_segment will return GST_FLOW_OK if the current segment is still valid, so it is OK to advance the segment_index to segments_count (not a valid index in the segment array) and return GST_FLOW_OK.

Please reverse the changes introduced in https://bugzilla.gnome.org/show_bug.cgi?id=751850