crypto/x509: don't fail on optional auth key id fields

If a certificate contains an AuthorityKeyIdentifier extension that
lacks the keyIdentifier field, but contains the authorityCertIssuer
and/or the authorityCertSerialNumber fields, don't return an error and
continue parsing.

Fixes #46854

Change-Id: I82739b415441f639a722755cc1f449d73078adfc
Reviewed-on: https://go-review.googlesource.com/c/go/+/331689


Trust: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>