fix range input in chart data source dialog

Change-Id: I9c139c6298d25c7a7072d0d0538f1945518d36f2
Reviewed-on: https://gerrit.libreoffice.org/13402
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>