ui/dbus: fix leak on message filtering

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>