gst/qtdemux/qtdemux.c: put back 'segment start<=stop' change that was mystically reverted by the last commit

Original commit message from CVS:
* gst/qtdemux/qtdemux.c: (gst_qtdemux_activate_segment),
(qtdemux_parse_tree):
put back 'segment start<=stop' change that was mystically reverted by
the last commit