don't use temporary object

This avoids invalid memory access with fdo45268-1.xlsx

Change-Id: If141df14d7d747023c944c83c2d8957abe8b95ad
Reviewed-on: https://gerrit.libreoffice.org/9718
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>