Ensure site_options have been loaded in `bp_get_signup_allowed()`.

This function is sometimes run before `$bp->site_options` has been populated,
resulting in incorrect 'registration' values.

Fixes #6043.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9189 cdf35c40-ae34-48e0-9cc9-0c9da1808c22