svx: fix one more EXCEPTION_INT_DIVIDE_BY_ZERO
See https://crashreport.libreoffice.org/stats/signature/SvxShowCharSet::PixelToMapIndex(Point%20const%20&)
Change-Id: Ied5698fd47220fae54d72be263c3eeed5fb77911
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/134899
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
(cherry picked from commit
663abd7162c29d289f84781eac56ca2c28b3a356)
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/134909
Reviewed-by: Thorsten Behrens <thorsten.behrens@allotropia.de>
Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>
Tested-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>