htlcswitch: fix panic when receiving close req for unknown channel
htlcswitch: fix panic when receiving close req for unknown channel

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