Prior to this, we were smashing our own stack,
which caused the crypto/sha256 tests to fail.

Change-Id: I7dd94cf466d175b3be0cd65f9c4fe8b1223081fe
Reviewed-on: https://go-review.googlesource.com/12660Reviewed-by: Daniel Morsing <daniel.morsing@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>