Have get_current_user_id() return 0 when pluggable.php is not yet included (which brings the ability to set or get the current user).

fixes #25690 for trunk.

Built from https://develop.svn.wordpress.org/trunk@25929


git-svn-id: http://core.svn.wordpress.org/trunk@25888 1a063a9b-81f0-0310-95a4-ce76da25c4cd