Corrected a regression in validation of admin fieldsets with nested...

Corrected a regression in validation of admin fieldsets with nested definitions from r11744. Thanks to Davor Lučić for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@11752 bcc190cf-cafb-0310-a4f2-bffc1f526a37