mnt: Make sure we don't hit nil dereference on nested mount

Such case is actually a BUG but while we can resolve
the situation without real bug-on call lets walk in
a gentle way.

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