test/sanitytest.c: fix setitimer usage in timer disarmament

test/sanitytest.c: fix setitimer usage in timer disarmament

It is incorrect to call setitimer(which, NULL, NULL) to disarm a timer,
Linux just gives a pass on that;  properly provide a zeroed-out
structure instead.

Fixes: 760929f6ba18 "crypto/sleep.c: avoid returning early due to signal"
Reported-by: Neil Horman <nhorman@openssl.org>
Signed-off-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28215)