Minor AEAD patch cleanup

Minor AEAD patch cleanup

 * Remove stale function declaration.
   This slipped into the AEAD cipher modes patch, but the function is
   now implemented as a static function is ssl.c.
 * Add ASSERT() to ensure frame is not NULL.
 * Fix "ENCRYPT TO" log message in openvpn_encrypt_aead().

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1456016892-8671-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/11233
Signed-off-by: Gert Doering <gert@greenie.muc.de>