worker_free() did not clear worker->event->worker, leaving an event
that was still attached to a worker pointing at freed memory once the
worker was gone. manager_free() frees the workers (hashmap_free())
before it walks manager->events to process them, and event_free() then
writes event->worker->event = NULL into the already-freed worker.
Clear the back-pointer in worker_free(), mirroring event_free() which
already clears the worker's pointer. The link used to be torn down by
the event_free(worker->event) call in worker_free(), which was dropped
when events became reference counted.
The Event and Worker definitions move to udev-manager.h (next to
Manager) so the unit test can construct and free a bound worker/event
pair.
Follow-up for cb16d47f30
Luca Boccassi
7eb7d46f97