[1.2.X] Fixed #14754 -- corrected using an aggregate in an F expressions when...

[1.2.X] Fixed #14754 -- corrected using an aggregate in an F expressions when that queryset is later used in a subquery.  Thanks to master for the patch. Backport of [14681].

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14682 bcc190cf-cafb-0310-a4f2-bffc1f526a37