Logout: Prevent SQL error on logout if already logged out in a different window/tab.

logout.php

@@ -28,6 +28,7 @@ include "root.php";
 require_once "resources/require.php";
 
 //check for login return preference
+	if ($_SESSION["user_uuid"] != '') {
 		if ($_SESSION['login']['destination_last']['boolean'] == 'true') {
 			if ($_SERVER['HTTP_REFERER'] != '') {
 				//convert to relative path
@@ -101,6 +102,7 @@ require_once "resources/require.php";
 				$db->exec(check_sql($sql));
 				unset($sql);
 		}
+	}
 
 //redirect the user to the index page
 	header("Location: ".PROJECT_PATH."/login.php");