core: Defer extracting the script until we're sure we'll use it

We were extracting the output script for all outputs, and discarding
them immediately again if they were not P2WSH outputs which are the
ones of interest to us. This patch move the extraction until after we
have determined it is useful, and so we should save a couple thousand
`tal()` and `tal_free()` calls.

Changelog-Changed: lightningd: Speed up blocksync by not parsing unused parts of the transactions

lightningd/chaintopology.c

@@ -942,9 +942,11 @@ static void topo_update_spends(struct chain_topology *topo, struct block *b)
 
 static void topo_add_utxos(struct chain_topology *topo, struct block *b)
 {
+	const u8 *script;
+	struct bitcoin_outpoint outpoint;
+	const struct bitcoin_tx *tx;
 	for (size_t i = 0; i < tal_count(b->full_txs); i++) {
-		const struct bitcoin_tx *tx = b->full_txs[i];
-		struct bitcoin_outpoint outpoint = { b->txids[i], 0 };
+		tx = b->full_txs[i];
 
 		for (size_t n = 0; n < tx->wtx->num_outputs; n++) {
 			if (tx->wtx->outputs[n].features
@@ -955,9 +957,11 @@ static void topo_add_utxos(struct chain_topology *topo, struct block *b)
 			if (!amount_asset_is_main(&amt))
 				continue;
 
-			const u8 *script = bitcoin_tx_output_get_script(tmpctx, tx, n);
-			if (is_p2wsh(script, NULL)) {
+			if (bitcoin_tx_output_script_is_p2wsh(tx, n)) {
+				outpoint.txid = b->txids[i];
 				outpoint.n = n;
+				script =
+				    bitcoin_tx_output_get_script(tmpctx, tx, n);
 				wallet_utxoset_add(topo->ld->wallet, &outpoint,
 						   b->height, i, script,
 						   amount_asset_to_sat(&amt));