Fixes a problem with the user creation ("User already existed" even if the...

Fixes a problem with the user creation ("User already existed" even if the user don't exists.) Thanks to Phil Reinhardt for pointing this out.

lib/user.php

@@ -345,9 +345,8 @@ class OC_User {
 	 * @return boolean
 	 */
 	public static function userExists($uid){
-		static $user_exists_checked = null;
-		if (!is_null($user_exists_checked)) {
-			return $user_exists_checked;
+		if (!is_null($user_exists_checked)) {
+			return $user_exists_checked;
 		}
 		foreach(self::$_usedBackends as $backend){
 			$result=$backend->userExists($uid);