parser fails to open old zecwallet wallet.dat

This wallet pre-exists orchard. that might be the problem

```
}
The application panicked (crashed).
Message:  called `Result::unwrap()` on an `Err` value: Custom { kind: InvalidInput, error: "Unable to deserialize a valid Orchard FullViewingKey from bytes" }
Location: zexcavator-lib/src/parser.rs:18
```