Dereferencing an empty boost::optional is dereferencing a this
nullptr, simply copy it instead.

Regression from

commit 0a18c1dc
CommitDate: Fri Dec 21 18:52:31 2018 +0100

    pass ScAddress around using boost::optional

Change-Id: Id676b2d84d324b989259688069d4860de5559ebc
Reviewed-on: https://gerrit.libreoffice.org/72605Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>