lwp/iomgr.c: avoid off-by-one out-of-bounds access

Avoid accessing sigDelivered[NSIG] and sigEvents[NSIG], which are
out-of-bounds by one.

Reviewed-on: http://gerrit.openafs.org/8883
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit bd850e5b980d34236ee98d16b9717860b63af306)

Change-Id: I859c654b8d35484aeeb029432c69848846335fb8
Reviewed-on: http://gerrit.openafs.org/9516
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>