NameNode should use NameNode.getRemoteUser() to log audit event to avoid possible NPE

private void logAuditEvent(boolean succeeded, String cmd, String src,
    String dst, FileStatus stat) throws IOException {
  if (isAuditEnabled() && isExternalInvocation()) {
    logAuditEvent(succeeded, Server.getRemoteUser(), Server.getRemoteIp(),
        cmd, src, dst, stat);
  }
}

// the ugi may be null.
private void logAuditEvent(boolean succeeded,
    UserGroupInformation ugi, InetAddress addr, String cmd, String src,
    String dst, FileStatus status) {
  final String ugiStr = ugi.toString();
  ...
}