Sign up activation error ignores root domain in redirection URL

[johngirvin]

In a multisite install, if there is an error processing a user activation from a link containing a key, eg:
​http://localhost.localdomain/content/activate?key=fd5f825062214b5c

...the user is redirected to BP_ACTIVATE_SLUG but without the root domain prefix, ie: /activate. This results in a 404 error.

Cause:
bp_core_activate_signup does not declare $bp as global, so $bp->root_domain is empty when it is used to build the redirection URL.

[DJPaul]

Affects multi-site installs only. Simple fix declaring the global, as per ticket description.

[johnjamesjacoby]

(In [2934]) Fixes #2257