[1.1.X] Fixed #11465: Ensure nonexistent pks enterd in an admin raw id field do not

cause a server error. Thanks for report and initial patch sacre@wp.pl. 

Backport of r12648 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12649 bcc190cf-cafb-0310-a4f2-bffc1f526a37