Required custom profile fields, are not required when a registered user accesses the registration form (solution to disable form for logged in users)

Based on MDL-42138.

Here is a point-by-point overview:
0. Moodle 2.8.2
1. E-mail based self-registration is enabled.
2. There is a custom 'register on the website' link added to the main menu. It points to /login/signup.php
3. There are a bunch of custom user profile fields added. All are set as required, to display on the signup page and as visible to user.

Now the scenario:
1. A user visits the website.
2. A user logs in/is logged in.
3. A user goes to the registration form (either from a link somewhere on the website or by manually adding /login/signup.php). A user fills the form while logged.
4. The required custom fields are not marked as required. The red asterisk is not displayed.
5. A user is able to proceed with the registration without filling in all the necessary information.