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).

Merges [25929] to the 3.7 branch.

fixes #25690.

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git-svn-id: http://core.svn.wordpress.org/branches/3.7@25889 1a063a9b-81f0-0310-95a4-ce76da25c4cd