tsocket/bsd: more correctly check if the cached tevent_fd is still valid

tsocket/bsd: more correctly check if the cached tevent_fd is still valid

I some cases the pointer value of tevent_context is the same again,
if we do something like:

	ev1 = tevent_context_init();
	...
	fde = tevent_add_fd(ev1, fd, TEVENT_FD_READ...);
	...
	talloc_free(ev1);
	...
	ev2 = tevent_context_init();

	if (ev1 == ev2) {
		/* this can happen! */
	}

	if (tevent_fd_get_flags(fde) == 0) {
		/* this is always true */
	}

But the "talloc_free(ev1)" will set fde->event_ctx to NULL
and tevent_fd_get_flags() will always return 0.

metze