Fix sign mismatch warning.
Fix sign mismatch warning.

swansontec/libbitcoin,GroestlCoin/libgroestlcoin,swansontec/libbitcoin,swansontec/libbitcoin,GroestlCoin/libgroestlcoin,swansontec/libbitcoin,GroestlCoin/libgroestlcoin,GroestlCoin/libgroestlcoin