[1.0.X] Changed r10672 to not falsely error out when using generic inlines.

The bug was picked up by the tests already, but only if run against a
backend that supports referential integrity.

Backport of r10732 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@10736 bcc190cf-cafb-0310-a4f2-bffc1f526a37