If the buffer is NULL, do not even try to attach it, and risk a Wayland
protocol error which would be fatal to us.
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Martin Peres <martin.peres@mupuf.org>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
https://gitlab.freedesktop.org/xorg/xserver/-/issues/1156