tty: Fix tty inheritance in case if zero sid fetched

In case if at checkpoint time we've fetched zero sid
associated with tty peer it doesn't obligatory means
that we've met a hangig up terminal, in particular
zero sid may be obtained if the slave peer do not
belong to the session the task run in but inherited
from the parent. Thus if --shell-job passed we should
handle such situation gracefully.

[ https://bugzilla.openvz.org/show_bug.cgi?id=2405

 ]

Reported-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>