crypto/internal/fips140/nistec: make SetBytes constant time
Similarly to CL 648035, SetBytes doesn't need to be constant time for
the uses we make of it in the standard library (ECDH and ECDSA public
keys), but it doesn't cost much to make it constant time for users of
the re-exported package, or even just to save the next person from
convincing themselves that it's ok for it not to be constant time.
Change-Id: I6a6a465622a0de08d9fc71db75c63185a82aa54a
Reviewed-on: https://go-review.googlesource.com/c/go/+/650579
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org> Reviewed-by: Michael Knyszek <mknyszek@google.com> Reviewed-by: Roland Shoemaker <roland@golang.org>