fix for undefined index notice in stats when there's no referer

-/stats.php

@@ -4,7 +4,7 @@ function record_stats($db, $url_id) {
 	$stmt = $db->prepare('INSERT INTO '.DB_PREFIX.'url_stats (url_id, ip_address, referer, created_on) VALUES(?,?,?,?)');
 	$stmt->bindValue(1, $url_id);
 	$stmt->bindValue(2, $_SERVER['REMOTE_ADDR']);
-	$stmt->bindValue(3, $_SERVER['HTTP_REFERER']);
+	$stmt->bindValue(3, isset($_SERVER['HTTP_REFERER']) ? $_SERVER['HTTP_REFERER'] : null);
 	$stmt->bindValue(4, date('Y-m-d H:i:s'));
 	$stmt->execute();
 }