Fix heap-buffer-overflow in CI fuzzing tests

Fix heap-buffer-overflow in CI fuzzing tests

The ASN1_STRING is not supposed to be used as a
zero-terminated string.  Therefore we need to check
the string length explicitly and use memcmp instead of
strcmp in ossl_x509_check_cert_time.

Fixes a regression introduced by #28623

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28962)