Can I use a buffer overflow exploit with this code, or is it secure?

gistfile1.c

int truncate_and_call(fn_ptr *fns, int index, char *user_string)
{
  char buf[64];
  // Truncate supplied string
  strncpy(buf, user_string, sizeof(buf) - 1);
  buf[sizeof(buf) - 1] = '\0';
  return fns[index](buf);
}