Prevent a crash when adding an institution with the same name as an existing one. Fixes #1851.

htdocs/admin/users/institutions.php

@@ -250,6 +250,12 @@ else {
     $smarty->assign('institutions', $institutions);
 }
 
+function institution_validate(Pieform $form, $values) {
+    if (!$form->get_error('name') && record_exists('institution', 'name', $values['name'])) {
+        $form->set_error('name', get_string('institutionnamealreadytaken', 'admin'));
+    }
+}
+
 function institution_submit(Pieform $form, $values) {
     global $SESSION, $institution, $add, $instancearray;
 

htdocs/lang/en.utf8/admin.php

@@ -256,6 +256,7 @@ $string['deleteinstitutionconfirm'] = 'Are you really sure you wish to delete th
 $string['institutionaddedsuccessfully'] = 'Institution added successfully. Please configure an authentication plugin for this institution.';
 $string['institutiondeletedsuccessfully'] = 'Institution deleted successfully';
 $string['institutionname'] = 'Institution name';
+$string['institutionnamealreadytaken'] = 'This institution name is already taken';
 $string['institutiondisplayname'] = 'Institution display name';
 $string['institutionupdatedsuccessfully'] = 'Institution updated successfully';
 $string['registrationallowed'] = 'Registration allowed?';