client: removed assert(client->fd)>=0

Since client->fd==-1 has become our "expired" flag, it may already be
-1 when client_close() is called.  Don't assert that it is still
non-negative, and call client_set_expired() instead.

src/client.c

@@ -230,15 +230,13 @@ static void client_close(struct client *client)
 {
 	struct sllnode *buf;
 
-	assert(client->fd >= 0);
-
-	xclose(client->fd);
-
 	assert(num_clients > 0);
 	assert(!list_empty(&clients));
 	list_del(&client->siblings);
 	--num_clients;
 
+	client_set_expired(client);
+
 	if (client->cmd_list) {
 		free_cmd_list(client->cmd_list);
 		client->cmd_list = NULL;