Missing return value check

[novalis]

https://github.com/LedgerHQ/app-tezos/blob/6793ac50d3ee82d39dffce8d838b683916f768fb/src/apdu_hmac.c#L35

Here, the return value of generate_key_pair is not checked, but elsewhere (e.g. here), it is.

Presumably, annotating it with __attribute__((warn_unused_result)) would help.