Only attempt email link sign-on if tokens exist

If the `view.php` page is requested and an auth token is not present in the
URL, do not attempt to sign the user on since it will always fail and will
result in a false "Failed login attempt" warning to the system log.
Merge pull request #763 from greezybacon/issue/unknown-user

Reviewed-By: Peter Rotich <peter@osticket.com>