paymod: Add invariant verification for constraints on shadowroute

This was highlighted in #3851, so I added an assertion. After the rewrite in
the next commit we would simply skip if any of the constraints were not
maintained, but this serves as the canary in the coalmine, so we don't paper over.

plugins/libplugin-pay.c

@@ -1784,6 +1784,11 @@ static struct command_result *shadow_route_listchannels(struct command *cmd,
 	const jsmntok_t *sattok, *delaytok, *basefeetok, *propfeetok, *desttok,
 	    *channelstok, *chan;
 
+	/* Check the invariants on the constraints between payment and modifier. */
+	assert(d->constraints.cltv_budget <= p->constraints.cltv_budget / 4);
+	assert(amount_msat_greater_eq(p->constraints.fee_budget,
+				      d->constraints.fee_budget));
+
 	channelstok = json_get_member(buf, result, "channels");
 	json_for_each_arr(i, chan, channelstok) {
 		u64 v = pseudorand(UINT64_MAX);