Add check to prevent the use of IP addresses as SNI hostnames

[new-sashok724]

Fixes #3676 (kind of).

When I attempt to add my mail server using a literal IPv6 address, I get the following error:

Screenshot

The error occurs because the constructor of javax.net.ssl.SNIHostName fails to parse the hostname. This behavior is consistent with RFC 6066, which explicitly prohibits using literal IPv4 or IPv6 addresses as SNI hostnames.

This PR fixes the problem by adding the corresponding check before setting the SNI hostname for the SSL/TLS connection.
The other problems mentioned in the original issue are no longer relevant, since the server settings validator was recently updated to accept literal IPv6 addresses.

[cketti]

Thanks for the pull request ❤️

Can you please move the IP address check outside of setSniHost() so we end up with

if (HostNameUtils.INSTANCE.isLegalIPAddress(host) == null) {
    setSniHost(socketFactory, sslSocket, host);
}

[cketti]

Thanks 👍