Fix an unaligned access on OpenBSD/sparc64

Fix an unaligned access on OpenBSD/sparc64

The pointer to the packet content doesn't seem to be word-aligned,
resulting in a SIGBUS when accessing it as a pointer to struct ip that
contains bit fields.

Replace with struct openvpn_iphdr and OPENVPN_IPH_GET_VER, which only
does a one byte access and thus isn't affected by alignement.
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <87ink3vpcs.fsf@ritchie.wxcvbn.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14769.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>