issue: No Ticket For checkStaffPerm()

This addresses an issue where there may be no `$ticket` available which
makes the `checkStaffPerm()` check fatally fail. This adds a check for
`$ticket` before checking the permissions. If no Ticket exists then we
set `Access Denied` error.

scp/tickets.php

@@ -37,7 +37,7 @@
          $errors['err']=sprintf(__('%s: Unknown or invalid ID.'), __('ticket'));
     elseif($_REQUEST['number'] && !($ticket=Ticket::lookup(array('number' => $_REQUEST['number']))))
          $errors['err']=sprintf(__('%s: Unknown or invalid number.'), __('ticket'));
-     elseif(!$ticket->checkStaffPerm($thisstaff)) {
+     elseif(!$ticket || !$ticket->checkStaffPerm($thisstaff)) {
          $errors['err']=__('Access denied. Contact admin if you believe this is in error');
          $ticket=null; //Clear ticket obj.
      }