txprepare: make output finding map for withdraw_tx variable len

The number of outputs got updated, but the map used to calculate the
change output's location did not (still assumes only one output). This
patch fixes this to make the output map a variable size.

Changelog-Fixed: JSON API: `txprepare` no longer crashes when more than two outputs are specified

common/withdraw_tx.c

@@ -19,24 +19,37 @@ struct bitcoin_tx *withdraw_tx(const tal_t *ctx,
 			       int *change_outnum)
 {
 	struct bitcoin_tx *tx;
+	int output_count;
 
 	tx = tx_spending_utxos(ctx, chainparams, utxos, bip32_base,
 			       !amount_sat_eq(change, AMOUNT_SAT(0)),
 			       tal_count(outputs));
 
-	bitcoin_tx_add_multi_outputs(tx, outputs);
+	output_count = bitcoin_tx_add_multi_outputs(tx, outputs);
+	assert(output_count == tal_count(outputs));
 
 	if (!amount_sat_eq(change, AMOUNT_SAT(0))) {
-		const void *map[2];
-		map[0] = int2ptr(0);
-		map[1] = int2ptr(1);
+		/* Add one to the output_count, for the change */
+		output_count++;
+
+		const void *map[output_count];
+		for (size_t i = 0; i < output_count; i++)
+			map[i] = int2ptr(i);
+
 		bitcoin_tx_add_output(tx, scriptpubkey_p2wpkh(tmpctx, changekey),
 				      change);
+
+		assert(tx->wtx->num_outputs == output_count);
 		permute_outputs(tx, NULL, map);
+
+		/* The change is the last output added, so the last position
+		 * in the map */
 		if (change_outnum)
-			*change_outnum = ptr2int(map[1]);
+			*change_outnum = ptr2int(map[output_count - 1]);
+
 	} else if (change_outnum)
 		*change_outnum = -1;
+
 	permute_inputs(tx, (const void **)utxos);
 	elements_tx_add_fee_output(tx);
 	assert(bitcoin_tx_check(tx));

tests/test_wallet.py

@@ -265,7 +265,6 @@ def test_deprecated_txprepare(node_factory, bitcoind):
     l1.rpc.call('txprepare', {'destination': addr, 'satoshi': Millisatoshi(amount * 100)})
 
 
-@pytest.mark.xfail
 def test_txprepare_multi(node_factory, bitcoind):
     amount = 10000000
     l1 = node_factory.get_node(random_hsm=True)