SEC-1 says: “The component privateKey is the private key defined to be
the octet string of length ⌊log₂(n)/8⌋ (where n is the order of the
curve)”.

Previously the code for parsing ECC private keys would panic (on
non-amd64) when the private was too long. It would also pass a too-short
private key to crypto/elliptic, possibly resulting in undesirable
behaviour.

This change makes the parsing function handle both too much and too
little padding because GnuTLS does the former and OpenSSL did the latter
until 30cd4ff294252c4b6a4b69cbef6a5b4117705d22. It also causes
serialisation to pad private keys correctly.

Fixes #13699

Change-Id: If9c2faeaeb45af8a4d7770d784f3d2633e7f8290
Reviewed-on: https://go-review.googlesource.com/18094
Run-TryBot: Adam Langley <agl@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>