Don't render the screen reader shortcut 'Log Out' link in the toolbar when the user is not logged in.

Merges [24442] to the 3.5 branch.

props SergeyBiryukov.
fixes #23178.



git-svn-id: https://core.svn.wordpress.org/branches/3.5@24443 1a063a9b-81f0-0310-95a4-ce76da25c4cd