rpcserver: use the first hop pubkey as the dest to the htlcSwitch

This commit modifies the sendpayment command slightly to use the pub
key of the *first* hop within the route as the destination within the
htlcPacket sent to the htlcSwitch.

Previously, since only single hop was implemented, the final
destination would be set within the htlcPkt, causing the route to fail
as the switch doesn’t have a direct link to the dest in the multi-hop
case.

rpcserver.go

@@ -510,7 +510,11 @@ func (r *rpcServer) SendPayment(paymentStream lnrpc.Lightning_SendPaymentServer)
 				RedemptionHashes: [][32]byte{rHash},
 				OnionBlob:        sphinxPacket,
 			}
-			destAddr := wire.ShaHash(fastsha256.Sum256(nextPayment.Dest))
+			firstHopPub, err := hex.DecodeString(path[1].String())
+			if err != nil {
+				return err
+			}
+			destAddr := wire.ShaHash(fastsha256.Sum256(firstHopPub))
 			htlcPkt := &htlcPacket{
 				dest: destAddr,
 				msg:  htlcAdd,