nss: refuse previously loaded certificate from file

... when we are not asked to use a certificate from file
size_t Curl_nss_version(char *buffer, size_t size)
{
  return snprintf(buffer, size, "NSS/%s", NSS_VERSION);
}

size_t Curl_nss_version(char *buffer, size_t size)
{
  return snprintf(buffer, size, "NSS/%s", NSS_VERSION);
}
