Null-terminate hex string in uuid_new()

  const char *s = uuid_string;

  for (size_t i = 0; i < sizeof(bt_uuid_t); ++i, s+=2) {

    char buf[2];

    char buf[3] = {0};

    buf[0] = s[0];

    buf[1] = s[1];

    uuid->uu[i] = strtoul(buf, NULL, 16);