Security
   * Fix a double-free that happened after mbedtls_ssl_set_session() or
     mbedtls_ssl_get_session() failed with MBEDTLS_ERR_SSL_ALLOC_FAILED
     (out of memory). After that, calling mbedtls_ssl_session_free()
     and mbedtls_ssl_free() would cause an internal session buffer to
     be free()'d twice.