zdtm: check posix_timer with sival_ptr = NULL

Here is a regresion test cae for

commit 1934e1a9638cdb8cd546cd0499fe7804317593d8
Author: Andrey Vagin <avagin@openvz.org>
Date:   Tue Feb 18 16:53:00 2014 +0400

    posix-timer: take into account that sival_ptr can be NULL (v3)

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