[secp256k1] ecmult_gen: Make code consistent with comment

Summary:
This also fixes a typo in the comment.

This is a partial backport of secp256k1#988
https://github.com/bitcoin-core/secp256k1/pull/988/commits/8ae18f1ab3dce4c487bab75c2f0cdf4fe311b318
Depends on D19384

Test Plan: ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D19385