audited using ineffassign tool from
github.com/gordonklaus/ineffassign
go generate does not generate any changes

Change-Id: Iabbec9ec1aae39081289d503d79fd7b4caadf17b
GitHub-Last-Rev: acd17cce410e9c68ce3c87b5546261be9153e3ea
GitHub-Pull-Request: golang/crypto#70
Reviewed-on: https://go-review.googlesource.com/c/155942Reviewed-by: Filippo Valsorda <filippo@golang.org>