Unverifiable external inputs not rejected in CoinJoin signing

When signing a CoinJoin transaction, the device is expected to reject any external input it cannot verify — one carrying neither a signature nor an ownership proof. The check that identified such inputs only recognised them when the relevant fields were entirely absent, so a malicious host could present an input in a form that escaped both the rejection and any subsequent signature verification. The classification has been corrected so that all unverifiable external inputs are rejected as intended.