Backend null in epoll during musl CI run

[SeanTAllen]

During an unrelated CI run, we hit an assertion during the pony_check/multiple_for_all test:

2023-09-07T13:20:38.8846124Z 282 tests started, 281 complete: pony_check/multiple_for_all started
2023-09-07T13:20:38.8851519Z [LWP 4479 exited]
2023-09-07T13:20:38.8873829Z /__w/ponyc/ponyc/src/libponyrt/asio/epoll.c:467: pony_asio_event_unsubscribe: Assertion `b != NULL` failed.
2023-09-07T13:20:38.8874426Z 
2023-09-07T13:20:38.8874550Z Backtrace functionality not available.
2023-09-07T13:20:38.8881455Z [LWP 4480 exited]
2023-09-07T13:20:38.8881752Z 
2023-09-07T13:20:38.8882286Z Thread 4 "stdlib-debug" received signal SIGABRT, Aborted.

from the code:

  asio_backend_t* b = ponyint_asio_get_backend();
  pony_assert(b != NULL);

I suspect this might have happened during shutdown. Otherwise it really makes no sense.