Unit tests output notices from bbp_current_author_ip()

[johnjamesjacoby]

Starting to setup PHPUnit, I'm getting notices from bbp_current_author_ip() because of our direct and unchecked use of $_SERVER['REMOTE_ADDR']

PHP Notice:  Undefined index: REMOTE_ADDR in /includes/users/functions.php on line 146

[johnjamesjacoby]

This can safely be fixed with an empty() check to start. In the event there's no REMOTE_ADDR I propose we force the IP address to '0.0.0.0' so at least there's an indication something strange occurred.

[johnjamesjacoby]

In 5609:

In bbp_current_author_ip() check $_SERVER['REMOTE_ADDR'] before attempting to parse it.

If it is empty (more likely for unit tests, but could be other oddities) we set it to '0.0.0.0' to do two things:

• Indicate to keymasters that something is amiss
• Allow bbp_current_author_ip() to proceed normally for the end-user

Fixes #2750.