fix: issue #355

[pipobscure]

The napi_external that maintains the watch on a file is closured into the stop function returned by fsevents.watch.
When that napi_external is garbage collected, the callback that was passed in is released and becomes uncallable. When it is called depsite this, the napi_env that is passed in is NULL which is intended by N-API as a signal that the function is no longer usable.

I ignored it in the past, because I always retain the stop function. However a test-script like the one provided provokes this. When garbage collection happens, the napi_external is cleared. However there may still be events in the pipeline. And that in turn triggers the crash in #355 .

I am now handling the napi_env as NULL signal properly by just dropping the event. In consequence, if you do not retain the stop function event reporting simply stops. And this will happen fairly quickly.

fixes #355

[pipobscure]

Just waiting for CI. Then merge and release.

[impowski]

Waiting for this one 🙏