channeldb: add some missing nil checks
channeldb: add some missing nil checks

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