Fix warning

htdocs/main.inc.php

@@ -713,7 +713,7 @@ if (!defined('NOLOGIN')) {
 		// If in demo mode, we check we go to home page through the public/demo/index.php page
 		if (!empty($dolibarr_main_demo) && $_SERVER['PHP_SELF'] == DOL_URL_ROOT.'/index.php') {  // We ask index page
 			if (empty($_SERVER['HTTP_REFERER']) || !preg_match('/public/', $_SERVER['HTTP_REFERER'])) {
-				dol_syslog("Call index page from another url than demo page (call is done from page ".$_SERVER['HTTP_REFERER'].")");
+				dol_syslog("Call index page from another url than demo page (call is done from page ".(empty($_SERVER['HTTP_REFERER']) ? '' : $_SERVER['HTTP_REFER']).")");
 				$url = '';
 				$url .= ($url ? '&' : '').($dol_hide_topmenu ? 'dol_hide_topmenu='.$dol_hide_topmenu : '');
 				$url .= ($url ? '&' : '').($dol_hide_leftmenu ? 'dol_hide_leftmenu='.$dol_hide_leftmenu : '');