Avoid use of uninitialized in tlsext_sni_server_parse()

Avoid use of uninitialized in tlsext_sni_server_parse()

If the hostname is too long, tlsext_sni_is_valid_hostname() will fail
without having initialized *is_ip. As a result, the garbage value could
lead to accepting (but otherwise ignoring) overlong and possibly invalid
hostnames without erroring in tlsext_sni_server_parse().

ok inoguchi jsing