This is a followup change to #13111 for filtering out IPv6 literals and
absolute FQDNs from being as the SNI values.

Updates #13111.
Fixes #14404.

Change-Id: I09ab8d2a9153d9a92147e57ca141f2e97ddcef6e
Reviewed-on: https://go-review.googlesource.com/19704Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>