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.

Author: stephanedelprat

lib/Saml2/LogoutRequest.php

@@ -150,6 +150,8 @@ public function getRequest($deflate = null)
      * @param string|DOMDocument $request Logout Request Message
      *
      * @return string ID
+     *
+     * @throws OneLogin_Saml2_Error
      */
     public static function getID($request)
     {
@@ -158,6 +160,13 @@ public static function getID($request)
         } else {
             $dom = new DOMDocument();
             $dom = OneLogin_Saml2_Utils::loadXML($dom, $request);
+            
+            if (false === $dom) {
+                throw new OneLogin_Saml2_Error(
+                    "XML is invalid",
+                    OneLogin_Saml2_Error::SAML_LOGOUTREQUEST_INVALID
+                );
+            }
         }
 
         $id = $dom->documentElement->getAttribute('ID');