nss: refuse previously loaded certificate from file

... when we are not asked to use a certificate from file
int Curl_nss_seed(struct Curl_easy *data)
{
  /* make sure that NSS is initialized */
  return !!Curl_nss_force_init(data);
}

int Curl_nss_seed(struct Curl_easy *data)
{
  /* make sure that NSS is initialized */
  return !!Curl_nss_force_init(data);
}
