ui/dbus: fix leak on message filtering

A filter function that wants to drop a message should return NULL, in
which case it must also unref the message itself.

Fixes: fa88b85de ("ui/dbus: filter out pending messages when scanout")

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-ID: <20241008125028.1177932-4-marcandre.lureau@redhat.com>
(cherry picked from commit 244d52ff736fefc3dd364ed091720aa896af306d)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

ui/dbus-listener.c

@@ -1001,6 +1001,7 @@ dbus_filter(GDBusConnection *connection,
     serial = g_dbus_message_get_serial(message);
     if (serial <= ddl->out_serial_to_discard) {
         trace_dbus_filter(serial, ddl->out_serial_to_discard);
+        g_object_unref(message);
         return NULL;
     }