Fix array_values() warning when using Tree Behavior under certain

situations.

In TreeBehavior::_setParent(), if the $Model->find('first') returned
false, then array_values() would throw a warning.

cake/libs/model/behaviors/tree.php

@@ -808,11 +808,16 @@ function _setParent(&$Model, $parentId = null, $created = false) {
 			$this->__sync($Model, $edge - $node[$left] + 1, '+', 'BETWEEN ' . $node[$left] . ' AND ' . $node[$right], $created);
 			$this->__sync($Model, $node[$right] - $node[$left] + 1, '-', '> ' . $node[$left], $created);
 		} else {
-			$parentNode = array_values($Model->find('first', array(
+			$values = $Model->find('first', array(
 				'conditions' => array($scope, $Model->escapeField() => $parentId),
 				'fields' => array($Model->primaryKey, $left, $right),
 				'recursive' => $recursive
-			)));
+			));
+
+			if ($values === false) {
+				return false;
+			}
+			$parentNode = array_values($values);
 
 			if (empty($parentNode) || empty($parentNode[0])) {
 				return false;