[1.1.X] Fixed #11183 - BaseForm init leaves pointers pointing back to base_fields

  
Thanks to margieroginski for the report

Backport of [12733] from trunk


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12736 bcc190cf-cafb-0310-a4f2-bffc1f526a37