io: fix a overflow bug
io: fix a overflow bug

It's introduced by 8fd675b9db1f6ff097c5e052072166d709ef7440 .

groonga/groonga,groonga/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,groonga/groonga,groonga/groonga,groonga/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga,kenhys/groonga