Do not process `BP_Group_Extension::setup_access_settings()` if no group ID is available.

If no group ID is available, the `BP_Group_Extension::setup_access_settings()`
method should not do its checks.  This is to avoid running DB queries
unnecessarily.  (See #4785.)

Fixes #5812.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8827 cdf35c40-ae34-48e0-9cc9-0c9da1808c22