faces/totp: avoid displaying when key is invalid

faces/totp: avoid displaying when key is invalid

Fixes a division by zero bug caused by calling getCodeFromTimestamp
without having initialized the TOTP library with a secret first.
This was happening because the face calls totp_display on activation,
meaning the validity of the secret was not checked since this is
done in the generate function.

Now the validity of the key is determined solely by the size
of the current decoded key. A general display function checks it
and decides whether to display the code or just the error message.

The size of the current decoded key is initialized to zero
on watch face activation, ensuring fail safe operation.

Tested-by: Matheus Afonso Martins Moreira <matheus.a.m.moreira@gmail.com>
Tested-on-hardware-by: madhogs <59648482+madhogs@users.noreply.github.com>
Signed-off-by: Matheus Afonso Martins Moreira <matheus.a.m.moreira@gmail.com>
GitHub-Pull-Request: https://github.com/joeycastillo/Sensor-Watch/pull/385