fix: not appending to buffer on multiple ops

[dj8yfo]

This affects the behaviour when signing multiple tx-s WITHOUT quitting near app between operations:
Below are signatures of the very same message:

• ledger prior to fix

  • first after entering app: 8f38d3db42410b03a63110f58a0c84eaf39a22dbe4e9ca866b406347342e897fa181fd0f9dbfcd0b1d4bbec1369f0f30bce3fa393a034670c8ad2c2cee24af02
  • second after enter: 71a89d556194d61d49691dfdc0f3921dcdf2039c0995999b8502e43b96ea830dc00d6f53870dfc0d0d382f9a9d3908f4b8525cedc9e05408a37bff009b42fa0d
  • third after enter: throws error SW_BUFFER_OVERFLOW 0x6990

• ledger after fix

  • first after entering app: 8f38d3db42410b03a63110f58a0c84eaf39a22dbe4e9ca866b406347342e897fa181fd0f9dbfcd0b1d4bbec1369f0f30bce3fa393a034670c8ad2c2cee24af02
  • second after enter: 8f38d3db42410b03a63110f58a0c84eaf39a22dbe4e9ca866b406347342e897fa181fd0f9dbfcd0b1d4bbec1369f0f30bce3fa393a034670c8ad2c2cee24af02
  • third after enter: 8f38d3db42410b03a63110f58a0c84eaf39a22dbe4e9ca866b406347342e897fa181fd0f9dbfcd0b1d4bbec1369f0f30bce3fa393a034670c8ad2c2cee24af02

[dj8yfo]

@tdejoigny-ledger please take a look

[tdejoigny-ledger]

hello @dj8yfo , do you have a discord account ?

[dj8yfo]

#32 is fast forward extension of this pr, as it depends on signing context reset on overflow in this line,
when first attempting to sign a too long message normally.

[dj8yfo]

the same effect as of this pr, is achieved in #32 by commit

[tdejoigny-ledger]

hello @dj8yfo, I noticed that you were making some contributions to the Near app. It could be good if you can join the dedicated Near channel on our discord, it will be easier to discuss about your PRs and the app process.

[dj8yfo]

@tdejoigny-ledger , sure.
Can you post a link here?
I'll create a pr to attach it to repo's README, as a nice place for cache instead of a local one on my machine.

upd: probably not relevant here, as the community appears not to be public by default