SLO reads wrong buffer when unmarshalling (crewjam#561)

Author: arjen-ag5

service_provider.go

@@ -1554,7 +1554,7 @@ func (sp *ServiceProvider) ValidateLogoutResponseRedirect(queryParameterData str
 	}
 
 	doc := etree.NewDocument()
-	if err := doc.ReadFromBytes(rawResponseBuf); err != nil {
+	if err := doc.ReadFromBytes(gr); err != nil {
 		retErr.PrivateErr = err
 		return retErr
 	}