Fixed bug with the internal structure of a certain type of trees

The internal structure is invalid, in case a tree height differs for more then one level. Here is an example dump of a table, revealing the problem:

CREATE TABLE IF NOT EXISTS `Test` (
  `id` int(11) NOT NULL,
  `root_value` int(11) DEFAULT NULL,
  `level_value` int(11) DEFAULT NULL,
  `left_value` int(11) DEFAULT NULL,
  `right_value` int(11) DEFAULT NULL,
  `label` varchar(255) NOT NULL,
  `created_at` datetime NOT NULL,
  `updated_at` datetime DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `INDEX_level_value` (`level_value`),
  KEY `INDEX_right_value` (`right_value`),
  KEY `INDEX_root_value` (`root_value`),
  KEY `INDEX_left_value` (`left_value`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `Test` (`id`, `root_value`, `level_value`, `left_value`, `right_value`, `label`, `created_at`, `updated_at`) VALUES
	(1351, 1351, 0, 1, 16, 'root', '2013-10-31 09:49:10', '2013-10-31 09:49:54'),
	(1352, 1351, 1, 2, 3, 'Kategorie 1', '2013-10-31 09:49:10', NULL),
	(1353, 1351, 1, 4, 5, 'Kategorie 2', '2013-10-31 09:49:10', NULL),
	(1354, 1351, 1, 6, 11, 'Kategorie 3', '2013-10-31 09:49:10', '2013-10-31 09:49:54'),
	(1355, 1351, 1, 12, 13, 'Kategorie 4', '2013-10-31 09:49:10', '2013-10-31 09:49:54'),
	(1356, 1351, 1, 14, 15, 'Kategorie 5', '2013-10-31 09:49:10', '2013-10-31 09:49:54'),
	(1401, 1351, 2, 7, 10, 'level', '2013-10-31 09:49:42', '2013-10-31 09:49:54'),
	(1402, 1351, 3, 8, 9, 'another level', '2013-10-31 09:49:54', '2013-10-31 09:49:54');