htlcswitch/hop: explicitly signal final hop from sphinx packet

Previously, we were using nextChanID to determine whether a hop
payload is for the final recipient. This is no longer suitable in a
route-blinding world where intermediate hops are allowed to have zero
nextChanID TLVs (as this information is provided to forwarding nodes
in their encrypted data). This commit updates payload reading to use
the signal provided by sphinx that we are on the last packet, rather
than implying it from the contents of a hop.

htlcswitch/hop/iterator.go

@@ -90,9 +90,10 @@ func (r *sphinxHopIterator) HopPayload() (*Payload, error) {
 	// Otherwise, if this is the TLV payload, then we'll make a new stream
 	// to decode only what we need to make routing decisions.
 	case sphinx.PayloadTLV:
-		return NewPayloadFromReader(bytes.NewReader(
-			r.processedPacket.Payload.Payload,
-		))
+		return NewPayloadFromReader(
+			bytes.NewReader(r.processedPacket.Payload.Payload),
+			r.processedPacket.Action == sphinx.ExitNode,
+		)
 
 	default:
 		return nil, fmt.Errorf("unknown sphinx payload type: %v",