secp256k1_fe_sqrt: Verify that the arguments don't alias.

Summary: This is a backport of secp256k1's PR551

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4976