Updated lightdm-gtk-greeter fixes CVE-2014-0979

Updated lightdm-gtk-greeter package fixes security vulnerability: lightdm-gtk-greeter uses the lightdm-gobject API incorrectly and does not handle lightdm_greeter_get_authentication_user() returning NULL when the username of the previous authentication is invalid resulting in a NULL pointer dereference in start_authentication(). This constitutes a local denial of service which can be triggered by any unprivileged attacker requiring the intervention of an administrator to restart lightdm (CVE-2014-0979).