Bug: initial_owner_utxo default might fail if taken by --issuance_utxo

[Sjors]

In asset.rs the following logic is brittle:

        let issuance_utxo: OutPoint = match matches.value_of("issuance_utxo") {
            Some(utxo) => {
                let parts: Vec<&str> = utxo.split(":").collect();

                OutPoint {
                    txid: Sha256dHash::from_hex(parts[0]).unwrap(),
                    vout: parts[1].parse().unwrap(),
                }
            }
            None => *unspent_utxos_outpoints[0].clone()
        };

        let initial_owner_utxo: OutPoint = match matches.value_of("initial_owner_utxo") {
            Some(utxo) => {
                let parts: Vec<&str> = utxo.split(":").collect();

                OutPoint {
                    txid: Sha256dHash::from_hex(parts[0]).unwrap(),
                    vout: parts[1].parse().unwrap(),
                }
            }
            None => *unspent_utxos_outpoints[1].clone()
        };

It picks the first unused UTXO if --issuance_utxo is not given. That's fine. But then it picks the second unused UTXO if --initial_owner_utxo is not set. That should lead to problems if --issuance_utxo happened to have just snatched that UTXO.

Unless there's some magic I'm missing.