channeld: fix crash with pre-TLV peers.

send_tlvs is NULL if no special features are supported, and peer
sets `next_to_send` anyway:

```
0x5ed1c6719538 peer_reconnect
channeld/channeld.c:5205
0x5ed1c6719dab init_channel
channeld/channeld.c:5959
0x5ed1c6719f04 main
channeld/channeld.c:6005
```

Backport: v24.08
Fixes: ElementsProject#7486
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

channeld/channeld.c

@@ -5158,7 +5158,7 @@ static void peer_reconnect(struct peer *peer,
 	} else if (pending_updates(peer->channel, LOCAL, true)
 		   || pending_updates(peer->channel, REMOTE, true)) {
 		status_debug("No upgrade: pending changes");
-	} else {
+	} else if (send_tlvs && recv_tlvs) {
 		const struct tlv_channel_reestablish_tlvs *initr, *ninitr;
 		const u8 *type;