src: use more explicit return type in Sign::SignFinal()

Using the non-indexed variant of `std::get<>` broke Travis CI.
Also, this allows us to be a bit more concise when returning
from `SignFinal()` due to some error condition.

Refs: https://github.com/nodejs/node/pull/23427

PR-URL: https://github.com/nodejs/node/pull/23779


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>