gst/schedulers/gstoptimalscheduler.c: Also ref/unref decoupled elements before iterating the group since they are not...

Original commit message from CVS:
* gst/schedulers/gstoptimalscheduler.c: (add_to_group),
(remove_from_group), (schedule_group), (normalize_group),
(gst_opt_scheduler_iterate):
Also ref/unref decoupled elements before iterating the
group since they are not added to the list of elements.