Change Fatal Error to Exception
Change Fatal Error to Exception

`OneLogin_Saml2_Utils::loadXML` returns `false` if input is not correct XML.

Which leads to a Fatal Error :

> Call to a member function getAttribute() on null in **/vendor/onelogin/php-saml/lib/Saml2/LogoutRequest.php:163

With this fix, an Exception is thrown.
onelogin/php-saml,stayallive/php-saml