[secp256k1] fix tests.c in the count == 0 case

Summary: This is a backport of secp256k1's PR529

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4954