gst/gstghostpad.c: Unlinking from a pad without a target is now a perfectly valid case which should NOT raise an asse...

Original commit message from CVS:
* gst/gstghostpad.c: (gst_ghost_pad_do_unlink):
Unlinking from a pad without a target is now a perfectly valid case
which should NOT raise an assertion.
This case would happen if a linked ghostpad its target set to NULL after
it was previously linked.