Do not pass invalid file descriptor to FD_ISSET()

Currently there is a race condition between the main thread and the
workers threads. The main thread sets nslcd_serversocket to -1 without
ensuring that all worker threads are stopped, giving them the window of
opportunity to pass the now invalid fd to FD_ISSET(). This results in
SIGBUS on musl libc.

Closing the file descriptor is enough. I've also dropped close() in
exithandler() to prevent misleading logs. The OS will close the socket
anyway.

nslcd/nslcd.c

@@ -221,13 +221,6 @@ static void sig_handler(int signum)
 /* do some cleaning up before terminating */
 static void exithandler(void)
 {
-  /* close socket if it's still in use */
-  if (nslcd_serversocket >= 0)
-  {
-    if (close(nslcd_serversocket))
-      log_log(LOG_WARNING, "problem closing server socket (ignored): %s",
-              strerror(errno));
-  }
   /* remove existing named socket */
   if (unlink(NSLCD_SOCKET) < 0)
   {
@@ -918,7 +911,6 @@ int main(int argc, char *argv[])
               i, strerror(errno));
   /* close server socket to trigger failures in threads waiting on accept() */
   close(nslcd_serversocket);
-  nslcd_serversocket = -1;
   /* if we can, wait a few seconds for the threads to finish */
 #ifdef HAVE_PTHREAD_TIMEDJOIN_NP
   ts.tv_sec = time(NULL) + 3;