Fix signed/unsigned mismatch.
Fix signed/unsigned mismatch.

chriseth/solidity-doc-test,shahankhatch/solidity,ruchevits/solidity,douglas-larocca/solidity,arkpar/solidity,LianaHus/solidity,vaporry/solidity,debris/solidity,ruchevits/solidity,ruchevits/solidity,ruchevits/solidity,ethers/solidity,gluk256/solidity,chriseth/solidity