Adjust limits (for STAX)

[andreibancioiu]

Allocation of tx_hash_context.current_value (bounded by MAX_VALUE_LEN):

• https://github.com/multiversx/mx-ledger-nano/blob/relayed-v3-support/src/sign_tx_hash.h#L40

While enc_len is bounded by MAX_DISPLAY_DATA_SIZE:

• https://github.com/multiversx/mx-ledger-nano/blob/relayed-v3-support/src/parse_tx.c#L241

For STAX, MAX_DISPLAY_DATA_SIZE = 256, thus memmove(encoded, tx_hash_context.current_value, enc_len); fails to correctly copy the data (destination buffer is too short). This leads to an error when decoding it as base64 (within verify_data).

For Nano S, MAX_DISPLAY_DATA_SIZE = 64, issue not present.

Fix: ensure that MAX_DISPLAY_DATA_SIZE <= MAX_VALUE_LEN on all devices.