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.

rpcserver.go

@@ -4374,6 +4374,15 @@ func (r *rpcServer) extractPaymentIntent(rpcPayReq *rpcPaymentRequest) (rpcPayme
 		payIntent.cltvDelta = uint16(r.cfg.Bitcoin.TimeLockDelta)
 	}
 
+	// 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.cltvDelta, true,
+	)
+	if err != nil {
+		return payIntent, err
+	}
+
 	// If the user is manually specifying payment details, then the payment
 	// hash may be encoded as a string.
 	switch {