Could happen with a malformed subformat string like "[<0-0".

Change-Id: I8eaab2bcb469d91432a41dc349060c273a6575a4
(cherry picked from commit f52aa4d1)
Reviewed-on: https://gerrit.libreoffice.org/16939Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>