revocation: fix nil pointer panic in shachain constructor
revocation: fix nil pointer panic in shachain constructor

bryanvu/lnd,LightningNetwork/lnd,ddustin/lnd,AndrewSamokhvalov/lnd,AndrewSamokhvalov/lnd,LightningNetwork/lnd,aakselrod/lnd,aakselrod/lnd,ddustin/lnd,BitfuryLightning/lnd,bryanvu/lnd,BitfuryLightning/lnd