fix overflow limit
fix overflow limit

lontivero/NBitcoin,dangershony/NStratis,HermanSchoenfeld/NBitcoin,thepunctuatedhorizon/BrickCoinAlpha.0.0.1,bitcoinbrisbane/NBitcoin,you21979/NBitcoin,NicolasDorier/NBitcoin,MetacoSA/NBitcoin,MetacoSA/NBitcoin,stratisproject/NStratis