routing: Fix possible infinite loop on first hop misleading hint

Add ignore condition to additional edges that connect to self. These
edges are already known and avoiding these hints protect the payment
from malformed channel ids which could lead to infinite loop.

Fixes lightningnetwork#6169.

Co-authored-by: lsunsi <lsunsi@pm.me>

routing/integrated_routing_test.go

@@ -4,9 +4,11 @@ import (
 	"fmt"
 	"testing"
 
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/btcutil"
 	"github.com/davecgh/go-spew/spew"
 	"github.com/lightningnetwork/lnd/lnwire"
+	"github.com/lightningnetwork/lnd/zpay32"
 	"github.com/stretchr/testify/require"
 )
 
@@ -234,6 +236,34 @@ var mppTestCases = []mppSendTestCase{
 	},
 }
 
+// TestBadFirstHopHint tests that a payment with a first hop hint with an
+// invalid channel id still works since the node already knows its channel and
+// doesn't need hints.
+func TestBadFirstHopHint(t *testing.T) {
+	t.Parallel()
+
+	ctx := newIntegratedRoutingContext(t)
+
+	onePathGraph(ctx.graph)
+
+	sourcePubKey, _ := btcec.ParsePubKey(ctx.source.pubkey[:])
+
+	hopHint := zpay32.HopHint{
+		NodeID:                    sourcePubKey,
+		ChannelID:                 66,
+		FeeBaseMSat:               0,
+		FeeProportionalMillionths: 0,
+		CLTVExpiryDelta:           100,
+	}
+
+	ctx.routeHints = [][]zpay32.HopHint{{hopHint}}
+
+	ctx.amt = lnwire.NewMSatFromSatoshis(100)
+	_, err := ctx.testPayment(1)
+
+	require.NoError(t, err, "payment failed")
+}
+
 // TestMppSend tests that a payment can be completed using multiple shards.
 func TestMppSend(t *testing.T) {
 	for _, testCase := range mppTestCases {