add fallback to system-hostname for faulty http-clients not setting 'Host' in the request

Signed-off-by: Michael Kaufmann <d00p@froxlor.org>

lib/Froxlor/UI/Panel/UI.php

@@ -92,6 +92,17 @@ class UI
 	 */
 	public static function sendHeaders()
 	{
+		if (empty($_SERVER['HTTP_HOST'])) {
+			if (!self::$install_mode) {
+				// fallback to set hostname in settings
+				$_SERVER['HTTP_HOST'] = Settings::Get('system.hostname');
+			} else {
+				// bad request
+				http_response_code(400);
+				exit();
+			}
+		}
+
 		session_set_cookie_params([
 			'lifetime' => self::$install_mode ? 7200 : 600, // will be renewed based on settings in lib/init.php
 			'path' => '/',