Ledger: Fix incorrect lenght of command INS_PREFIX_HASH

[grydz]

Using version 0.16.0.0 of the monero-wallet-cli, an error appears when trying to transfer funds to more than one address on the Ledger Nano S/X:

Error: unexpected error: Wrong Device Status: 0x6701 (UNKNOWN), EXPECTED 0x9000 (SW_OK), MASK 0xffff

This is the due to an incorrect assignment of buffer_send[4] (unsigned char representing LC) line 1482 to more than 255 which results to an incorrect lenght command.

   1     1    1    1    1     1     var
+-------------------------------------------+
| CLA | INS | P1 | P2 | LC | Opt | Add data |
+-------------------------------------------+
                        ^
                len(Opt + Add data)

It's fixed by setting len to be at most 254 (262-5-3) which the maximum lenght in Add data expected.

[moneromooo-monero]

Would it be even better to use BUFFER_SEND_SIZE-offset-1 (possibly off by one, but you get the idea) ?

[cslashm]

yes that good idea.
@grydz

[grydz]

Done.

[dEBRUYNE-1]

@grydz - Can you open an equivalent PR against master as well?

[grydz]

@grydz - Can you open an equivalent PR against master as well?

Here is the equivalent PR on master: #6597.

[dEBRUYNE-1]

Thanks!