Make bench_internal obey secp256k1_fe_sqrt's contract wrt aliasing.
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

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