Fix simple problem with substr where warning was being thrown

git-svn-id: https://svn.cakephp.org/repo/branches/1.2.x.x@7992 3807eeeb-6ff5-0310-8944-8be069107fe0

cake/libs/multibyte.php

@@ -966,6 +966,7 @@ function substr($string, $start, $length = null) {
 		}
 		$string = array_values($string);
 
+		$value = array();
 		for ($i = 0; $i < $length; $i++) {
 			$value[] = $string[$i];
 		}