unsigned long X509_issuer_and_serial_hash(X509 *a)
{
    unsigned long ret = 0;
    EVP_MD_CTX *ctx = EVP_MD_CTX_new();
    unsigned char md[16];
    char *f;

    if (ctx == NULL)
        goto err;
    f = X509_NAME_oneline(a->cert_info.issuer, NULL, 0);
    if (!EVP_DigestInit_ex(ctx, EVP_md5(), NULL))
        goto err;
    if (!EVP_DigestUpdate(ctx, (unsigned char *)f, strlen(f)))
        goto err;
    OPENSSL_free(f);
    if (!EVP_DigestUpdate
        (ctx, (unsigned char *)a->cert_info.serialNumber.data,
         (unsigned long)a->cert_info.serialNumber.length))
        goto err;
    if (!EVP_DigestFinal_ex(ctx, &(md[0]), NULL))
        goto err;
    ret = (((unsigned long)md[0]) | ((unsigned long)md[1] << 8L) |
           ((unsigned long)md[2] << 16L) | ((unsigned long)md[3] << 24L)
        ) & 0xffffffffL;
 err:
    EVP_MD_CTX_free(ctx);
    return ret;
}
The product dereferences a pointer that it expects to be valid but is NULL.