tlsdate-helper: fix SAN checking

Right now, SAN checking checks against the host we're opening a socket to
instead of the host we're actually trying to talk to, which is fine... as long
as we don't have a proxy. Note that this problem only manifests for hosts whose
CN is not equal to their hostname (so the default host of www.ptb.de is fine).

To observe the problem:
$ ssh -D 127.0.0.1:30000 somehost
$ tlsdate -H clients3.google.com -x socks5://127.0.0.1:30000
hostname verification failed for host 127.0.0.1!
child process failed in SSL handshake

With this fix, you instead see no output.

Signed-off-by: Elly Fong-Jones <ellyjones@chromium.org>