it's a COW type internally, so only one pointer big

Change-Id: I478f5aa6f84c87bedf6f450526e7f8f7c297e7c4
Reviewed-on: https://gerrit.libreoffice.org/65529
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>