rpcserver: enforce keysend minimum carrier HTLC amount

Fixes an issue where attempting to pay a below-dust carrier HTLC satoshi
amount would knock a channel offline.

Author: guggero

rpcserver.go

@@ -7787,6 +7787,16 @@ func (r *rpcServer) SendPayment(req *tchrpc.SendPaymentRequest,
 				"for keysend payment")
 		}
 
+		// We must enforce a minimum satoshi amount to make sure the
+		// carrier HTLC is above dust. This is handled by the traffic
+		// shaper for invoice-based payments, but for keysend payments
+		// we need to do it here.
+		if pReq.Amt < int64(rfqmath.DefaultOnChainHtlcSat) {
+			return fmt.Errorf("keysend payment satoshi amount "+
+				"must be greater than or equal to %d satoshis",
+				rfqmath.DefaultOnChainHtlcSat)
+		}
+
 		if len(req.PaymentRequest.Dest) == 0 {
 			return fmt.Errorf("destination node must be " +
 				"specified for keysend payment")