crypto/tls: don't send IP literals as SNI values.

https://tools.ietf.org/html/rfc6066#section-3 states:

  “Literal IPv4 and IPv6 addresses are not permitted in "HostName".”

However, if an IP literal was set as Config.ServerName (which could
happen as easily as calling Dial with an IP address) then the code would
send the IP literal as the SNI value.

This change filters out IP literals, as recognised by net.ParseIP, from
being sent as the SNI value.

Fixes #13111.

Change-Id: Ie9ec7acc767ae172b48c9c6dd8d84fa27b1cf0de
Reviewed-on: https://go-review.googlesource.com/16742


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