pkeyutl exit with 0 if the verification succeeded

If exiting non-zero, which not consistent with shell conventions,
the shells/scripts treat the cmd as failed.

CLA: trivial

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3213)