These processes don't have image files in a parent snapshot and crtools
should not fail in this case.

https://bugzilla.openvz.org/show_bug.cgi?id=2636

v2: return NULL from mem_snap_init, if a parent image is absent.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>