Avoid overflow in wakeup time computation

Avoid overflow in wakeup time computation

Time interval arithmetic can overflow especially when user
defined intervals are involved. E.g., see Trac #922.

Avoid this by reordering the arithmetic operation in
event_timeout_trigger(). Also avoid unnecessary casting of time
variable to int.

Time until wakeup is now calculated like:

time_t wakeup = (last - now) + delay

Here delay is of type int, but is +ve by construction. Time backtrack
protection in OpenVPN ensures (last - now) <= 0. Then the above
expression cannot overflow (provided time_t is at least as large
as int).

A similar expression in interval.h is also changed.

(This patch grew out of patch 168 by Steffan Karger.)

Trac: #922

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <1520316568-8983-1-git-send-email-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16634.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit f158c0e1df13ae1b697cdc7f189ddd1575a0c1aa)