-Wmaybe-uninitialized in ScTable::CopyColFiltered

apply same initialization as in ScTable::CopyRowFiltered

Change-Id: I9e8de6eaae3b3ad9453e5c06a88cfa93766becd9
Reviewed-on: https://gerrit.libreoffice.org/11263
Reviewed-by: Kohei Yoshida <libreoffice@kohei.us>
Tested-by: Kohei Yoshida <libreoffice@kohei.us>