Initialize field elements when resulting in infinity

Summary:

• Added test with additions resulting in infinity
• Clear field elements when writing infinity

This is a backport of secp256k1 PR699

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7611