Prevent websitePath from overwriting

The $GLOBALS['TL_CONFIG']['websitePath'] may be set in localconfig.php, in this case some Contao functions, like System::setCookie()* which still use $GLOBALS['TL_CONFIG']['websitePath'] use a wrong path, so no cookie will work in case of wrong cookie path.

* https://github.com/contao/core/blob/master/system/modules/core/library/Contao/System.php#L462