askrene-inform-channel: change order of operations

Change order of operations to ensure that askrene-inform-channel with
inform=constrained computes the correct upper bound on the liquidity:

    upperBound = reserves + amount - 1

With the previous order of operations in the case amount=0 we would get:

    upperBound = reserves

instead of
    upperBound = reserves - 1

Changelog-None

Signed-off-by: Lagrang3 <[email protected]>

Author: Lagrang3

plugins/askrene/askrene.c

@@ -1036,11 +1036,11 @@ static struct command_result *json_askrene_inform_channel(struct command *cmd,
 	case INFORM_CONSTRAINED:
 		/* It didn't pass, so minimal assumption is that reserve was all used
 		 * then there we were one msat short. */
-		if (!amount_msat_sub(amount, *amount, AMOUNT_MSAT(1)))
-			*amount = AMOUNT_MSAT(0);
 		if (!reserve_accumulate(askrene->reserved, scidd, amount))
 			return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
 					    "Amount overflow with reserves");
+		if (!amount_msat_sub(amount, *amount, AMOUNT_MSAT(1)))
+			*amount = AMOUNT_MSAT(0);
 		if (command_check_only(cmd))
 			return command_check_done(cmd);
 		c = layer_add_constraint(layer, scidd, time_now().ts.tv_sec,