Added zero address checks for transactions
Added zero address checks for transactions

pipermerriam/pyethereum,ethereum/pyethereum,karlfloersch/pyethereum,pipermerriam/pyethereum,holiman/pyethereum,shahankhatch/pyethereum,ckeenan/pyethereum,vaporry/pyethereum,karlfloersch/pyethereum,ethereum/pyethereum,shahankhatch/pyethereum,inzem77/pyethereum