int FuzzerTestOneInput(const uint8_t *buf, size_t len)
{
    const unsigned char *p = buf;
    unsigned char *der = NULL;

    X509 *x509 = d2i_X509(NULL, &p, len);
    if (x509 != NULL) {
        BIO *bio = BIO_new(BIO_s_null());
        /* This will load and print the public key as well as extensions */
        X509_print(bio, x509);
        BIO_free(bio);

        i2d_X509(x509, &der);
        OPENSSL_free(der);

        X509_free(x509);
    }
    ERR_clear_error();
    return 0;
}
The product dereferences a pointer that it expects to be valid but is NULL.