Fixed #12717 -- Corrected a problem with subqueries when using multidb...

Fixed #12717 -- Corrected a problem with subqueries when using multidb routing. Thanks to Jeff Balogh for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12755 bcc190cf-cafb-0310-a4f2-bffc1f526a37