crypto/x509: make verification of an empty certificate consistent across platforms.

crypto/x509: make verification of an empty certificate consistent across platforms.

Platform-specific verification needs the ASN.1 contents of a certificate
but that might not be provided if the Certificate was not created by
ParseCertificate. In order to avoid a panic on Windows, and to make
behaviour consistent across platforms, this change causes verification
to fail when the ASN.1 contents of a certificate are not available.

Fixes #12184

Change-Id: I4395d74934e675c179eaf4cded1094a756e478bb
Reviewed-on: https://go-review.googlesource.com/14053
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>