[secp256k1] Reduce usage of hardcoded size constant

Summary:
This is backport of PR463 by Thomas Snider

Depends on D1250

Test Plan:

make check

Reviewers: #bitcoin_abc, matiu, jasonbcox

Reviewed By: #bitcoin_abc, matiu, jasonbcox

Subscribers: matiu, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1255