X509_find_by_issuer_and_serial
signer = X509_find_by_issuer_and_serial(untrusted, ias->issuer, ias->serial);
return (X509_find_by_issuer_and_serial(p7->d.sign->cert,
signer = X509_find_by_issuer_and_serial(certs,
signer = X509_find_by_issuer_and_serial(included_certs,
X509 *X509_find_by_issuer_and_serial(STACK_OF(X509) *sk, const X509_NAME *name,