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/mock_graph_test.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"testing"
 
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/btcutil"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/lnwire"
@@ -13,7 +14,12 @@ import (
 
 // createPubkey return a new test pubkey.
 func createPubkey(id byte) route.Vertex {
-	pubkey := route.Vertex{id}
+	_, secpPub := btcec.PrivKeyFromBytes([]byte{id})
+
+	var bytes [33]byte
+	copy(bytes[:], secpPub.SerializeCompressed()[:33])
+
+	pubkey := route.Vertex(bytes)
 	return pubkey
 }