! prevent error log on empty topic (db error), plus allow a way to remove it.

Sources/Display.php

@@ -794,7 +794,7 @@ function Display()
 	$posters = array_unique($all_posters);
 
 	// Guests can't mark topics read or for notifications, just can't sorry.
-	if (!$user_info['is_guest'])
+	if (!$user_info['is_guest'] && !empty($messages))
 	{
 		$mark_at_msg = max($messages);
 		if ($mark_at_msg >= $topicinfo['id_last_msg'])