Memory leak in us_create_timer

[abrownsword]

struct us_poll_t *p = us_create_poll(loop, fallthrough, sizeof(struct us_internal_callback_t) + ext_size);
int timerfd = timerfd_create(CLOCK_REALTIME, TFD_NONBLOCK | TFD_CLOEXEC);
if (timerfd == -1) {
  return NULL;
}

Leaks p if the timer fails to be created.

[partyblob]

Fixed in #231