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:
c0bf69a791

Reviewed-by: Rich Salz <rsalz@openssl.org>

ssl/ssl_locl.h

@@ -1860,6 +1860,7 @@ __owur int ssl_set_peer_cert_type(SESS_CERT *c, int type);
 __owur int ssl_get_new_session(SSL *s, int session);
 __owur int ssl_get_prev_session(SSL *s, unsigned char *session, int len,
                          const unsigned char *limit);
+__owur SSL_SESSION *ssl_session_dup(SSL_SESSION *src, int ticket);
 __owur int ssl_cipher_id_cmp(const SSL_CIPHER *a, const SSL_CIPHER *b);
 DECLARE_OBJ_BSEARCH_GLOBAL_CMP_FN(SSL_CIPHER, SSL_CIPHER, ssl_cipher_id);
 __owur int ssl_cipher_ptr_id_cmp(const SSL_CIPHER *const *ap,