hardening: add safe FD_SET() wrapper openvpn_fd_set()

hardening: add safe FD_SET() wrapper openvpn_fd_set()

On many platforms (not Windows, for once), FD_SET() can write outside the
given fd_set if an fd >= FD_SETSIZE is given.  To make sure we don't do
that, add an ASSERT() to error out with a clear error message when this
does happen.

This patch was inspired by remarks about FD_SET() from Sebastian Krahmer
of the SuSE Security Team.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1456996968-29472-1-git-send-email-steffan.karger@fox-it.com>
URL: http://article.gmane.org/gmane.network.openvpn.devel/11285
Signed-off-by: Gert Doering <gert@greenie.muc.de>