worker: fix stream racing with terminate

`OnStreamAfterReqFinished` uses `v8::Object::Has` to check if it needs
to call `oncomplete`. `v8::Object::Has` needs to execute Javascript.
However when worker threads are involved, `OnStreamAfterReqFinished` may
be called after the worker thread termination has begun via
`worker.terminate()`. This makes `v8::Object::Has` return `Nothing`,
which triggers an assert.

This diff fixes the issue by simply defaulting us to `false` in the case
where `Nothing` is returned. This is sound because we can't execute
`oncomplete` anyway as the isolate is terminating.

Fixes: nodejs#38418

src/stream_base.cc

@@ -619,7 +619,7 @@ void ReportWritesToJSStreamListener::OnStreamAfterReqFinished(
     stream->ClearError();
   }
 
-  if (req_wrap_obj->Has(env->context(), env->oncomplete_string()).FromJust())
+  if (req_wrap_obj->Has(env->context(), env->oncomplete_string()).FromMaybe(false))
     async_wrap->MakeCallback(env->oncomplete_string(), arraysize(argv), argv);
 }