Fixed #14414 -- Improved contenttypes shortcut() view to check that the...

Fixed #14414 -- Improved contenttypes shortcut() view to check that the ContentType has a model_class(). Thanks, subsume

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13994 bcc190cf-cafb-0310-a4f2-bffc1f526a37