crypto/x509: return better error when a certificate contains no names.

Currently, if a certificate contains no names (that we parsed),
verification will return the confusing error:
    x509: certificate is valid for , not example.com.

This change improves the error for that situation.

Fixes #16834.

Change-Id: I2ed9ed08298d7d50df758e503bdb55277449bf55
Reviewed-on: https://go-review.googlesource.com/30152


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>