remove warning: ‘nIndex’ may be used uninitialized

Change-Id: I317c2f4409f556ab967e4f08caa99cffcfce26cc
Reviewed-on: https://gerrit.libreoffice.org/11876
Reviewed-by: Kohei Yoshida <libreoffice@kohei.us>
Tested-by: Kohei Yoshida <libreoffice@kohei.us>