client: check "expired" flag again in client_write_output()

When client_defer_output() aborts the connection to the client,
client_write_output() called client_write_deferred() anyway.  This
caused an assertion failure.  Fix it by checking for the "expired"
flag again after client_defer_output() returns.

src/client.c

@@ -776,6 +776,9 @@ static void client_write_output(struct client *client)
 		client_defer_output(client, client->send_buf,
 				    client->send_buf_used);
 
+		if (client_is_expired(client))
+			return;
+
 		/* try to flush the deferred buffers now; the current
 		   server command may take too long to finish, and
 		   meanwhile try to feed output to the client,