gst/gstelementfactory.c: Unref the factory after it was used the last time, not before.

gst/gstelementfactory.c: Unref the factory after it was used the last time, not before.

Original commit message from CVS:
* gst/gstelementfactory.c: (gst_element_factory_make):
Unref the factory after it was used the last time, not before.
* gst/gstindexfactory.c: (gst_index_factory_make):
Improve debugging a bit and don't leak a ref to the index factory with
each call.