Original commit message from CVS:
* gst/gstinterface.c: (gst_element_implements_interface):
Don't crash if the element supports the interface queried, but does
not implement GstImplementsInterface. Fixes#519584.
* tests/check/Makefile.am:
* tests/check/gst/.cvsignore:
* tests/check/gst/gstinterface.c:
Add unit test for the above.