Fix stack overflow error on multi-site

This is causing a major bug mentioned in this [issue](https://github.com/WP-API/WP-API/issues/509).

In multi-site, wp_authenticate_spam_check filter is run on authentication. This filter calls get_currentuserinfo which in turn calls the determine_current_user filter. This leads to infinite recursion and a stack overflow unless the current function is removed from the determine_current_user filter during authentication.