Fixed #8353 -- Corrected the case when adminform is not in the context when...

Fixed #8353 -- Corrected the case when adminform is not in the context when rendering {% prepopulated_fields_js %} in the admin. Thanks evan_schulz for the initial patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8421 bcc190cf-cafb-0310-a4f2-bffc1f526a37