routerrpc: fix sendpayment_v2 negative fee limit

Author: hieblmi

lnrpc/marshall_utils.go

@@ -23,6 +23,9 @@ var (
 	ErrSatMsatMutualExclusive = errors.New(
 		"sat and msat arguments are mutually exclusive",
 	)
+
+	// ErrNegativeAmt is returned when a negative amount is specified.
+	ErrNegativeAmt = errors.New("amount cannot be negative")
 )
 
 // CalculateFeeLimit returns the fee limit in millisatoshis. If a percentage
@@ -56,6 +59,10 @@ func UnmarshallAmt(amtSat, amtMsat int64) (lnwire.MilliSatoshi, error) {
 		return 0, ErrSatMsatMutualExclusive
 	}
 
+	if amtSat < 0 || amtMsat < 0 {
+		return 0, ErrNegativeAmt
+	}
+
 	if amtSat != 0 {
 		return lnwire.NewMSatFromSatoshis(btcutil.Amount(amtSat)), nil
 	}