In BP_Signup::activate(), make sure the user ID is defined before checking user_status

When Activating a signup from the Signups management screen, in case of an error, we need to first try to get the user id before checking his status.

Fixes #5559

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8294 cdf35c40-ae34-48e0-9cc9-0c9da1808c22