pulse-server: pending-sample: move reference counting

Move the reference counting from `pending_sample_free()`
into `on_sample_done()` so that the client's references
are managed in a single place.

The reason why `pending_sample_free()` cannot simply call
`client_unref()` is that `client_free()` may be called from
`manager_disconnect()` regardless of the reference count,
and, in turn, `pending_sample_free()` may be called,
which could then lead to a recursive call to `client_free()`.

src/modules/module-protocol-pulse/pending-sample.c

@@ -41,7 +41,5 @@ void pending_sample_free(struct pending_sample *ps)
 	spa_hook_remove(&ps->listener);
 	pw_work_queue_cancel(impl->work_queue, ps, SPA_ID_INVALID);
 
-	client->ref--;
-
 	sample_play_destroy(ps->play);
 }

src/modules/module-protocol-pulse/pulse-server.c

@@ -2344,9 +2344,9 @@ static void on_sample_done(void *obj, void *data, int res, uint32_t id)
 {
 	struct pending_sample *ps = obj;
 	struct client *client = ps->client;
+
 	pending_sample_free(ps);
-	if (client->ref <= 0)
-		client_free(client);
+	client_unref(client);
 }
 
 static void sample_play_done(void *data, int res)