dual-fund: we were double-broadcasting the funding tx

On a reconnect after a restart, node will re-broadcast the funding tx.
This isn't terrible but there's no reason to do it.

lightningd/dual_open_control.c

@@ -1707,6 +1707,7 @@ static void handle_peer_tx_sigs_msg(struct subd *dualopend,
 	struct lightningd *ld = dualopend->ld;
 	struct channel *channel = dualopend->channel;
 	struct channel_inflight *inflight;
+	bool had_sigs;
 
 	if (!fromwire_dualopend_funding_sigs(tmpctx, msg, &psbt)) {
 		channel_internal_error(channel,
@@ -1724,10 +1725,6 @@ static void handle_peer_tx_sigs_msg(struct subd *dualopend,
 		return;
 	}
 
-	/* Save that we've gotten their sigs. Sometimes
-	 * the peer doesn't send any sigs (no inputs), otherwise
-	 * we could just check the PSBT was finalized */
-	inflight->remote_tx_sigs = true;
 	psbt_txid(NULL, psbt, &txid, NULL);
 	if (!bitcoin_txid_eq(&txid, &inflight->funding->txid)) {
 		channel_internal_error(channel,
@@ -1756,13 +1753,21 @@ static void handle_peer_tx_sigs_msg(struct subd *dualopend,
 		return;
 	}
 	tal_wally_end(inflight->funding_psbt);
+
+	/* Save that we've gotten their sigs. Sometimes
+	 * the peer doesn't send any sigs (no inputs), otherwise
+	 * we could just check the PSBT was finalized */
+	had_sigs = inflight->remote_tx_sigs;
+	inflight->remote_tx_sigs = true;
 	wallet_inflight_save(ld->wallet, inflight);
 
 	/* It's possible we haven't sent them our (empty) tx-sigs yet,
 	 * but we should be sending it soon... */
 	if (psbt_finalize(cast_const(struct wally_psbt *,
 			  inflight->funding_psbt))
-	    && !inflight->tx_broadcast) {
+	    && !inflight->tx_broadcast
+	    && !had_sigs) {
+
 		inflight->tx_broadcast = true;
 
 		/* Saves the now finalized version of the psbt */