restore: prevent killing of nonpositive PIDs

I don't like surprises.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

cr-restore.c

@@ -877,6 +877,7 @@ out:
 				kill(root_item->pid.real, SIGKILL);
 		} else {
 			for_each_pstree_item(pi)
+				if (pi->pid.virt > 0)
 					kill(pi->pid.virt, SIGKILL);
 		}
 		return 1;