Previously the code didn't check the type and final five dashes of the
ending line of a PEM block.

Fixes #16335.

Change-Id: Ia544e8739ea738d767cfe56c8d46204214ec0b5a
Reviewed-on: https://go-review.googlesource.com/27391
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>