footnote labels must not contain line breaks

Change-Id: I00d334197b960c4ed17fdee3ea8a1c734cbfa4c1
Reviewed-on: https://gerrit.libreoffice.org/18292Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>
(cherry picked from commit 089d7f53)
Reviewed-on: https://gerrit.libreoffice.org/18297Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>