[1.2.X] Fixed #15349 - Bound FormSet produces bound empty_form

Thanks to hidde-jan for the report and patch.

Backport of [15614] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15615 bcc190cf-cafb-0310-a4f2-bffc1f526a37