multi: validate payment params at RPC layer

With this patch, we'll fail out earlier in the cycle in case of
some wonky parameters, and not leave zombie payments in the router
which currently are not cleaned up.

lnrpc/routerrpc/router_backend.go

@@ -238,6 +238,14 @@ func (r *RouterBackend) QueryRoutes(ctx context.Context,
 	if in.FinalCltvDelta != 0 {
 		finalCLTVDelta = uint16(in.FinalCltvDelta)
 	}
+
+	// Do bounds checking without block padding so we don't give routes
+	// that will leave the router in a zombie payment state.
+	err = routing.ValidateCLTVLimit(cltvLimit, finalCLTVDelta, false)
+	if err != nil {
+		return nil, err
+	}
+
 	cltvLimit -= uint32(finalCLTVDelta)
 
 	// Parse destination feature bits.
@@ -860,6 +868,15 @@ func (r *RouterBackend) extractIntentFromSendRequest(
 		payIntent.DestFeatures = features
 	}
 
+	// Do bounds checking with the block padding so the router isn't
+	// left with a zombie payment in case the user messes up.
+	err = routing.ValidateCLTVLimit(
+		payIntent.CltvLimit, payIntent.FinalCLTVDelta, true,
+	)
+	if err != nil {
+		return nil, err
+	}
+
 	// Check for disallowed payments to self.
 	if !rpcPayReq.AllowSelfPayment && payIntent.Target == r.SelfNode {
 		return nil, errors.New("self-payments not allowed")