lnwallet: fix panic on aux sig handling

Author: GeorgeTsagk

lnwallet/channel.go

@@ -5553,7 +5553,10 @@ func genHtlcSigValidationJobs(chanState *channeldb.OpenChannel,
 			// disk later.
 			sigType := htlcCustomSigType.TypeVal()
 			auxSig.WhenSome(func(sigB tlv.Blob) {
-				htlc.CustomRecords[uint64(sigType)] = sigB
+				if htlc.CustomRecords != nil {
+					htlc.CustomRecords[uint64(sigType)] =
+						sigB
+				}
 			})
 
 			auxVerifyJobs = append(auxVerifyJobs, auxVerifyJob)