channeld-df: actually check serial_id of input when setting sigs

We're about to totally upset the order that sigs are set on our PSBTs
for new channel opens, making it such that our peer's sigs may arrive
before ours do.

We can no longer rely on the 'set witness means this is our input' since
there's no guarantee that our input sigs have been added yet, so we
check the serial_id and only set the stack on their (odd) inputs.

channeld/channeld.c

@@ -2009,6 +2009,9 @@ static void handle_tx_sigs(struct peer *peer, const u8 *msg)
 	const struct witness_stack **ws;
 	const struct wally_tx *wtx;
 	size_t j = 0;
+	enum tx_role role = peer->channel->opener == REMOTE
+		? TX_INITIATOR : TX_ACCEPTER;
+
 
 	if (!fromwire_tx_signatures(tmpctx, msg, &cid, &txid,
 				    cast_const3(
@@ -2047,11 +2050,17 @@ static void handle_tx_sigs(struct peer *peer, const u8 *msg)
 	for (size_t i = 0; i < peer->psbt->num_inputs; i++) {
 		struct wally_psbt_input *in =
 			&peer->psbt->inputs[i];
+		u16 in_serial;
 		const struct witness_element **elem;
-		/* Really we should check serial parity, but we can
-		 * cheat and only check that the final witness
-		 * stack hasn't been set yet */
-		if (in->final_witness)
+
+		if (!psbt_get_serial_id(&in->unknowns, &in_serial)) {
+			status_broken("PSBT input %zu missing serial_id %s",
+				      i, type_to_string(tmpctx,
+							struct wally_psbt,
+							peer->psbt));
+			return;
+		}
+		if (in_serial % 2 != role)
 			continue;
 
 		if (j == tal_count(ws))