Incorrect implementations of hook_permission cause permissions page to whitescreen.

If any module implements hook_permission incorrectly, by not creating a nested array for each declared permission, then the permissions page gets a fatal error for an unsupported operand. The issue resides in the user_admin_permissions() function, which on line 690 loops through all permissions declared by module's hook_permissions implementation:

      foreach ($permissions as $perm => $perm_item) {
        // Fill in default values for the permission.
        $perm_item += array(
          'description' => '',
          'restrict access' => FALSE,
          'warning' => !empty($perm_item['restrict access']) ? t('Warning: Give to trusted roles only; this permission has security implications.') : '',
        );

The WSOD will occur if $perm_item is not an array. If it is not, then an unsupported operands fatal error occurs.

To correct this, the functions should check that $perm_item is an array before adding to it.