Fix errors with illegal string offsets.

If $_list becomes a string notice errors are triggered when trying
to do offsets in PHP 5.4

lib/Cake/Utility/Set.php

@@ -671,14 +671,17 @@ public static function insert($list, $path, $data = null) {
 			if (is_numeric($key) && intval($key) > 0 || $key === '0') {
 				$key = intval($key);
 			}
-			if ($i === $count - 1) {
+			if ($i === $count - 1 && is_array($_list)) {
 				$_list[$key] = $data;
 			} else {
 				if (!isset($_list[$key])) {
 					$_list[$key] = array();
 				}
 				$_list =& $_list[$key];
 			}
+			if (!is_array($_list)) {
+				return array();
+			}
 		}
 		return $list;
 	}