Make bench_internal obey secp256k1_fe_sqrt's contract wrt aliasing.
Make bench_internal obey secp256k1_fe_sqrt's contract wrt aliasing.

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

afk11/secp256k1,peterdettman/secp256k1,libbitcoin/secp256k1,bitcoin/secp256k1,bitcoin/secp256k1,evoskuil/secp256k1,afk11/secp256k1,pmienk/secp256k1,bitcoin/secp256k1,pmienk/secp256k1,evoskuil/secp256k1,libbitcoin/secp256k1,bitcoin-core/secp256k1,pmienk/secp256k1,libbitcoin/secp256k1,ElementsProject/secp256k1-zkp,pmienk/secp256k1,evoskuil/secp256k1,ElementsProject/secp256k1-zkp,afk11/secp256k1,evoskuil/secp256k1,evoskuil/secp256k1,peterdettman/secp256k1,afk11/secp256k1,bitcoin/secp256k1,bitcoin/secp256k1,libbitcoin/secp256k1,pmienk/secp256k1,afk11/secp256k1,libbitcoin/secp256k1,bitcoin-core/secp256k1