curl_gethostname.c: fix signed/unsigned comparison and avoid a double copy

  return -1;

#else

  int err = 0;

  char* dot = NULL;

  char hostname[HOSTNAME_MAX + 1];

  int err;

  char* dot;

#ifdef DEBUGBUILD

  /* Override host name when environment variable CURL_GETHOSTNAME is set */

  const char *force_hostname = getenv("CURL_GETHOSTNAME");

  if(force_hostname) {

    strncpy(hostname, force_hostname, sizeof(hostname));

    hostname[sizeof(hostname) - 1] = '\0';

    strncpy(name, force_hostname, namelen);

    err = 0;

  }

  else {

    name[0] = '\0';

    err = gethostname(name, namelen);

  }

  else

    err = gethostname(hostname, sizeof(hostname));

#else /* DEBUGBUILD */

  /* The call to system's gethostname() might get intercepted by the

     libhostname library when libcurl is built as a non-debug shared

     library when running the test suite. */

  err = gethostname(hostname, sizeof(hostname));

  name[0] = '\0';

  err = gethostname(name, namelen);

#endif

  if(err != 0)

  name[namelen - 1] = '\0';

  if(err)

    return err;

  /* Is the hostname fully qualified? */

  dot = strchr(hostname, '.');

  if(dot) {

    /* Copy only the machine name to the specified buffer */

    size_t size = dot - hostname;

    strncpy(name, hostname, namelen > size ? size : namelen);

    name[(namelen > size ? size : namelen) - 1] = '\0';

  }

  else {

    /* Copy the hostname to the specified buffer */

    strncpy(name, hostname, namelen);

    name[namelen - 1] = '\0';

  }

  /* Truncate domain, leave only machine name */

  dot = strchr(name, '.');

  if(dot)

    *dot = '\0';

  return 0;

#endif