Fixed #17506 -- Did a minor optimization in the sessions' database backend....

Fixed #17506 -- Did a minor optimization in the sessions' database backend. Thanks to FunkyBob for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17390 bcc190cf-cafb-0310-a4f2-bffc1f526a37