Fix PHP notice in groups_screen_group_admin_settings()

To support legacy group edit templates, we must check to see that
$_POST['group-invite-status'] is populated before sanitizing the value
against our status whitelist.

Fixes #5132

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7343 cdf35c40-ae34-48e0-9cc9-0c9da1808c22