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

ID: 0
signal: 26/          (null)
notify: signal/pid.5954
ClockID: 1

fscanf "%p" doesn't handle "(null)".

https://bugzilla.openvz.org/show_bug.cgi?id=2894



v2: make the original scanf be %d/%s and then additionally
    parse the obtained string
v3: don't use strstr

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