Incorrect spend authorization signatures returned by hardware wallet

[murisi]

The call get_spend_signature() returns incorrect spend signatures when the number of generated spend randomness parameters (equal to the number of calls to get_spend_randomness()) exceeds the number of spend descriptions (equal to the number of spend authorization signatures pushed). It seems that this is due to how the hardware wallet internally indexes the spend authorization signatures it is fetching here:

ledger-namada/app/src/nvdata.c

Line 140 in 5ed06a4

const uint8_t index = transaction_header.spendlist_len - transaction_header.spends_sign_index;

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[ainhoa-a]

Can be closed once #69 gets merged