Original commit message from CVS:
* gst/gstpad.c: (gst_pad_link_negotiate):
refuse to link if the link is not possible
* configure.ac:
* testsuite/Makefile.am:
* testsuite/negotiation/.cvsignore:
* testsuite/negotiation/Makefile.am:
* testsuite/negotiation/pad_link.c: (main):
add test that checks the above behaviour
