gst/gstelement.c: Refuse registering a pad template if another pad template with the same name already exists (#114715).

Original commit message from CVS:
2004-01-03  Ronald Bultje  <rbultje@ronald.bitfreak.net>

* gst/gstelement.c: (gst_element_class_add_pad_template):
Refuse registering a pad template if another pad template
with the same name already exists (#114715).