crypto/x509: use unsafe.Slice

crypto/x509: use unsafe.Slice

Change-Id: I40fdfbd5cfb9f5ccb80d55bca28ff9ba1ec490f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/435285
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>