Fix race condition in NewSessionTicket

If a NewSessionTicket is received by a multi-threaded client when
attempting to reuse a previous ticket then a race condition can occur
potentially leading to a double free of the ticket data.

CVE-2015-1791

This also fixes RT#3808 where a session ID is changed for a session already
in the client session cache. Since the session ID is the key to the cache
this breaks the cache access.

Parts of this patch were inspired by this Akamai change:
https://github.com/akamai/openssl/commit/c0bf69a791239ceec64509f9f19fcafb2461b0d3

Reviewed-by: Rich Salz <rsalz@openssl.org>
void SSL_SESSION_get0_ticket(const SSL_SESSION *s, unsigned char **tick,
                            size_t *len)
{
    *len = s->tlsext_ticklen;
    if (tick != NULL)
        *tick = s->tlsext_tick;
}

void SSL_SESSION_get0_ticket(const SSL_SESSION *s, unsigned char **tick,
                            size_t *len)
{
    *len = s->tlsext_ticklen;
    if (tick != NULL)
        *tick = s->tlsext_tick;
}
