A wallet that has patched the CVE-2020-14199 vulnerability will always
include a non-witness UTXO, even for witness inputs. In the signer, we
detect that the input we spend is a witness input and copy over the
TxOut to the witness UTXO field. Therefore it is possible that both UTXO
fields are set at the same time. We need to adjust the sanity checks
when adding a partial signature to account for that.
