l10ntools/source/export.cxx: add sanity check to aOutput.mSimple

Change-Id: I05074760f189dfc00ce14ec33002609a56797a6d
Reviewed-on: https://gerrit.libreoffice.org/11321
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>