User registration throws errors

Master 15.04

Initial assessment from Robert:
the problem is in lib/user.php function display_username(). somehow a $user variable is passed in that is missing $user->username. so I suspect that the $user variable is not a true $user object.

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Test scenario:

1. Set up a new institution and enable registration and "Confirm registration".
2. Register as new user and you'll get the error message below.

[WAR] 42 (lib/user.php:1196) Undefined property: stdClass::$username
Call stack (most recent first):

    log_message("Undefined property: stdClass::$username", 8, true, true, "/home/kristina/code/mahara/htdocs/lib/user.php", 1196) at /home/kristina/code/mahara/htdocs/lib/errors.php:441
    error(8, "Undefined property: stdClass::$username", "/home/kristina/code/mahara/htdocs/lib/user.php", 1196, array(size 2)) at /home/kristina/code/mahara/htdocs/lib/user.php:1196
    display_username(object(stdClass)) at /home/kristina/code/mahara/htdocs/lib/user.php:1042
    display_name(object(stdClass), object(stdClass)) at /home/kristina/code/mahara/htdocs/lib/user.php:731
    email_user(object(stdClass), null, "Request for registration at Mahara received", "Hi test, Thank you for requesting an account on M...", "<p>Hi test,</p> <p>Thank you for requesting an acc...") at /home/kristina/code/mahara/htdocs/auth/lib.php:2457
    auth_register_submit(object(Pieform), array(size 10)) at Unknown:0
    call_user_func_array("auth_register_submit", array(size 2)) at /home/kristina/code/mahara/htdocs/lib/pieforms/pieform.php:532
    Pieform->__construct(array(size 11)) at /home/kristina/code/mahara/htdocs/auth/lib.php:2228
    auth_generate_registration_form_js(array(size 11), array(size 1)) at /home/kristina/code/mahara/htdocs/register.php:204