routing: ensure we create a pruneViewSnapshot for route sessions

In this commit, we fix an existing bug that could at times lead to a
panic if a user manually crafts a route via SendToRoute, and that route
results in a payment error. The fix is simple: create the map even
though it won't be used in the sessions since the user is feeding the
router manual routes.

routing/missioncontrol.go

@@ -249,6 +249,7 @@ func (m *missionControl) NewPaymentSession(routeHints [][]HopHint,
 // used for things like channel rebalancing, and swaps.
 func (m *missionControl) NewPaymentSessionFromRoutes(routes []*Route) *paymentSession {
 	return &paymentSession{
+		pruneViewSnapshot: m.GraphPruneView(),
 		haveRoutes:        true,
 		preBuiltRoutes:    routes,
 		mc:                m,