admin/users/institutions.php tries to set institution expiry for 'mahara' no institution

[Andrew Nicols]

Shown in the apache error log:

[WAR] 5c (admin/users/institutions.php:403) Undefined index: expiry, referer: http://mahara.local/admin/users/institutions.php
Call stack (most recent first):, referer: http://mahara.local/admin/users/institutions.php
  * log_message("Undefined index: expiry", 8, true, true, "/home/nicols/mahara/htdocs/admin/users/institution...", 403) at /home/nicols/mahara/htdocs/lib/errors.php:446, referer: http://mahara.local/admin/users/institutions.php
  * error(8, "Undefined index: expiry", "/home/nicols/mahara/htdocs/admin/users/institution...", 403, array(size 9)) at /home/nicols/mahara/htdocs/admin/users/institutions.php:403, referer: http://mahara.local/admin/users/institutions.php
  * institution_submit(object(Pieform), array(size 33)) at Unknown:0, referer: http://mahara.local/admin/users/institutions.php
  * call_user_func_array("institution_submit", array(size 2)) at /home/nicols/mahara/htdocs/lib/pieforms/pieform.php:430, referer: http://mahara.local/admin/users/institutions.php
  * Pieform->__construct(array(size 5)) at /home/nicols/mahara/htdocs/lib/pieforms/pieform.php:170, referer: http://mahara.local/admin/users/institutions.php
  * Pieform::process(array(size 5)) at /home/nicols/mahara/htdocs/lib/pieforms/pieform.php:71, referer: http://mahara.local/admin/users/institutions.php
  * pieform(array(size 5)) at /home/nicols/mahara/htdocs/admin/users/institutions.php:335, referer: http://mahara.local/admin/users/institutions.php
, referer: http://mahara.local/admin/users/institutions.php

The expiry field is not added to the pieform, and therefore when an attempt is made to set it in institution_submit,
this fails as there is no $values['expiry'].

Attached patch moves the setting into an existing test for $institution != 'mahara'