mount: cgroup -- Fix nil dereference

In case if there is no CLONE_NEWCGROUP on root
namespace mask the @private member remains nil
and comparision in mounts_sb_equal leads to nil
dereference.

CC: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/mount.c

@@ -305,7 +305,9 @@ static bool mounts_sb_equal(struct mount_info *a, struct mount_info *b)
 			return false;
 	}
 
-	if (a->fstype->code == FSTYPE__CGROUP && strcmp(a->private, b->private))
+	if (a->fstype->code == FSTYPE__CGROUP &&
+	    a->private && b->private &&
+	    strcmp(a->private, b->private))
 		return false;
 
 	return a->s_dev == b->s_dev && !strcmp(a->source, b->source);