use index instead of an assumption of a constant

Author: JBlond

lib/Diff/Renderer/Html/Array.php

@@ -91,8 +91,8 @@ public function mb_substr_replace($string, $replacement, $start, $length=NULL) {
 		preg_match_all('/./us', (string)$string, $smatches);
 		preg_match_all('/./us', (string)$replacement, $rmatches);
 		if ($length === NULL) $length = mb_strlen($string);
-		array_splice($smatches[0], $start, $length, $rmatches[0]);
-		return join($smatches[0]);
+		array_splice($smatches['0'], $start, $length, $rmatches[0]);
+		return join($smatches['0']);
 	}
 
 	/**