Fixed #9989: fixed a subtle edge case where removing signals could break....

Fixed #9989: fixed a subtle edge case where removing signals could break. Thanks, ferringb. Backport of r10398 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@10399 bcc190cf-cafb-0310-a4f2-bffc1f526a37