Malformed GET_WALLET_PUBKEY causes app to halt

[apoelstra]

In https://github.com/LedgerHQ/blue-app-btc/blob/master/src/btchip_helpers.c#L220-L222 there is a THROW which is easy to trigger by sending a GET_WALLET_PUBKEY with BIP32 chain longer than 10. The app then halts and will refuse to answer future messages until the user exists and re-enters it.

This has a benefit, I can trigger it intentionally, e.g. if my PC application thinks it's being attacked and wants to quickly disable the dongle. But it would probably be better to have a dedicated message for this.

A more consistent behaviour would be to return BTCHIP_SW_INCORRECT_DATA to the caller.