gst/schedulers/gstoptimalscheduler.c: when removing an element from a group, we alway...

gst/schedulers/gstoptimalscheduler.c: when removing an element from a group, we always need to decrement the link cou...

Original commit message from CVS:
* gst/schedulers/gstoptimalscheduler.c: (remove_from_group),
(group_dec_link), (gst_opt_scheduler_pad_unlink):
when removing an element from a group, we always need to
decrement the link count that this group had with other
groups through the element.
added an extra assert to catch inconsistencies when decrementing
the link count.