group_user_access returns false when there is no logged-in user.  But
there are times when the code may still want to call this function on
a valid user id and group when there is no active user.

For example, when a logged-out user is looking at a public forum, we
still need to find out whether a poster has permission to publish a
particular image artefact in the post, and being able to use the
group_user_access function is handy in this case.

Change-Id: Id2ef045311fd05220b683a2a2dea4f12cef269f4
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>