+ /*
+ * Free internal session cache. However: the remove_cb() may reference
+ * the ex_data of SSL_CTX, thus the ex_data store can only be removed
+ * after the sessions were flushed.
+ * As the ex_data handling routines might also touch the session cache,
+ * the most secure solution seems to be: empty (flush) the cache, then
+ * free ex_data, then finally free the cache.
+ * (See ticket [openssl.org #212].)
+ */