Minor adjustments to WindowAgg startup cost code
This is a follow-on of
3900a02c9 containing some changes which I forgot
to commit locally before forming a patch with git format-patch.
Discussion: https://postgr.es/m/CAApHDvrB0S5BMv+0-wTTqWFE-BJ0noWqTnDu9QQfjZ2VSpLv_g@mail.gmail.com