Prevent websitePath from overwriting

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