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.

Built from https://develop.svn.wordpress.org/branches/3.7@25930


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

wp-includes/user.php

@@ -212,6 +212,8 @@ function count_many_users_posts( $users, $post_type = 'post', $public_only = fal
  * @return int The current user's ID
  */
 function get_current_user_id() {
+	if ( ! function_exists( 'wp_get_current_user' ) )
+		return 0;
 	$user = wp_get_current_user();
 	return ( isset( $user->ID ) ? (int) $user->ID : 0 );
 }