fix validation of notifcation by sender/from address

Reviewed-on: https://dev.sum7.eu/genofire/unified-push-xmpp/pulls/19

distributor/xmpp.go

@@ -113,7 +113,7 @@ func (s *XMPPService) message(msgHead stanza.Message, t xmlstream.TokenReadEncod
 		return nil
 	}
 	from := msgHead.From.String()
-	if settings := strings.Split(conn.Settings, ":"); len(settings) > 1 && settings[0] == from {
+	if settings := strings.Split(conn.Settings, ":"); !(len(settings) > 1 && settings[0] == from) {
 		log.WithField("from", from).Info("message not from gateway, that is no notification")
 		return nil
 	}