lnwire: enforce a max limit on an outpoint's index
lnwire: enforce a max limit on an outpoint's index

This commit implements the constraint on an outpoint’s index as defined
within the specification.

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