xwayland: Fix non-argb cursor conversion

From the bug: "What happens if bits->width is less than 8? :)"

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=103012


Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
(cherry picked from commit 97ac59b1)