X509_CRL_get_ext_by_NID
int i = X509_CRL_get_ext_by_NID(a, nid, -1);
if (X509_CRL_get_ext_by_NID(a, nid, i) != -1)
i = X509_CRL_get_ext_by_NID(b, nid, -1);
if (X509_CRL_get_ext_by_NID(b, nid, i) != -1)
int X509_CRL_get_ext_by_NID(const X509_CRL *x, int nid, int lastpos);