Core: match the `registration` route precisely.

Previously, you could enter the registration route with non-existent
parts, such as `/registration/test/`, and BuddyPress would serve up the
registration template.

By checking if `bp_current_action()` is set (the `/test/` part), we can
bail out of `bp_core_screen_signup()`. BuddyPress will correctly 404
these incorrect URLs.

Fixes #5772

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8658 cdf35c40-ae34-48e0-9cc9-0c9da1808c22