crypto/x509: check the private key passed to CreateCertificate

Unfortunately, we can't improve the function signature to refer to
crypto.PrivateKey and crypto.PublicKey, even if they are both
interface{}, because it would break assignments to function types.

Fixes #37845

Change-Id: I627f2ac1e1ba98b128dac5382f9cc2524eaef378
Reviewed-on: https://go-review.googlesource.com/c/go/+/224157


Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>