Make bench_internal obey secp256k1_fe_sqrt's contract wrt aliasing.

Summary:
Bench_internal was previously incorrect but wasn't detected by
-DVERIFY until PR #551.

This is a backport of secp256k1's PR591

Test Plan:

cmake -GNinja .. -DCMAKE_C_FLAGS=-DVERIFY
ninja bench-secp256k1-internal

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4986