Replace unaligned 16bit access to TCP MSS value with bytewise access

Replace unaligned 16bit access to TCP MSS value with bytewise access

TCP options are not always word-aligned, and accessing a 16bit value
at an odd memory address will cause a "bus error" crash on some
architectures, e.g. Linux/Sparc(64)

Trac #497

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <1440680402-96548-1-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10056
(cherry picked from commit 2e2a34181962b33d70c34c28dcb1e1977c2fd54e)