routing: fix index out of range panic

Make sure that we do not crash the node trying to access an invalid
position for an array (-1) in `BuildRoute`.

routing/router.go

@@ -2782,7 +2782,7 @@ func (r *ChannelRouter) BuildRoute(amt *lnwire.MilliSatoshi,
 	}
 
 	pathEdges, receiverAmt, err := getPathEdges(
-		senderAmt, unifiers, bandwidthHints, hops,
+		sourceNode, senderAmt, unifiers, bandwidthHints, hops,
 	)
 	if err != nil {
 		return nil, err
@@ -2879,8 +2879,8 @@ func getRouteUnifiers(source route.Vertex, hops []route.Vertex,
 
 // getPathEdges returns the edges that make up the path and the total amount,
 // including fees, to send the payment.
-func getPathEdges(receiverAmt lnwire.MilliSatoshi, unifiers []*edgeUnifier,
+func getPathEdges(source route.Vertex, receiverAmt lnwire.MilliSatoshi,
-	bandwidthHints *bandwidthManager,
+	unifiers []*edgeUnifier, bandwidthHints *bandwidthManager,
 	hops []route.Vertex) ([]*channeldb.CachedEdgePolicy,
 	lnwire.MilliSatoshi, error) {
 
@@ -2892,8 +2892,13 @@ func getPathEdges(receiverAmt lnwire.MilliSatoshi, unifiers []*edgeUnifier,
 	for i, unifier := range unifiers {
 		edge := unifier.getEdge(receiverAmt, bandwidthHints)
 		if edge == nil {
+			fromNode := source
+			if i > 0 {
+				fromNode = hops[i-1]
+			}
+
 			return nil, 0, ErrNoChannel{
-				fromNode: hops[i-1],
+				fromNode: fromNode,
 				position: i,
 			}
 		}

routing/router_test.go

@@ -3187,6 +3187,55 @@ func TestBuildRoute(t *testing.T) {
 	}
 }
 
+// TestGetPathEdges tests that the getPathEdges function returns the expected
+// edges and amount when given a set of unifiers and does not panic.
+func TestGetPathEdges(t *testing.T) {
+	t.Parallel()
+
+	const startingBlockHeight = 101
+	ctx := createTestCtxFromFile(t, startingBlockHeight, basicGraphFilePath)
+
+	testCases := []struct {
+		sourceNode     route.Vertex
+		amt            lnwire.MilliSatoshi
+		unifiers       []*edgeUnifier
+		bandwidthHints *bandwidthManager
+		hops           []route.Vertex
+
+		expectedEdges []*channeldb.CachedEdgePolicy
+		expectedAmt   lnwire.MilliSatoshi
+		expectedErr   string
+	}{{
+		sourceNode: ctx.aliases["roasbeef"],
+		unifiers: []*edgeUnifier{
+			{
+				edges:     []*unifiedEdge{},
+				localChan: true,
+			},
+		},
+		expectedErr: fmt.Sprintf("no matching outgoing channel "+
+			"available for node 0 (%v)", ctx.aliases["roasbeef"]),
+	}}
+
+	for _, tc := range testCases {
+		pathEdges, amt, err := getPathEdges(
+			tc.sourceNode, tc.amt, tc.unifiers, tc.bandwidthHints,
+			tc.hops,
+		)
+
+		if tc.expectedErr != "" {
+			require.Error(t, err)
+			require.ErrorContains(t, err, tc.expectedErr)
+
+			continue
+		}
+
+		require.NoError(t, err)
+		require.Equal(t, pathEdges, tc.expectedEdges)
+		require.Equal(t, amt, tc.expectedAmt)
+	}
+}
+
 // edgeCreationModifier is an enum-like type used to modify steps that are
 // skipped when creating a channel in the test context.
 type edgeCreationModifier uint8