afsmonitor: Use NULL, rather than (struct foo *)0

Reviewed-on: http://gerrit.openafs.org/9285
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit caabad98232557220f153461520fc0cf1c6978fb)

Change-Id: Ie1e0e1bb4dcae88f17c592b4ebccaef227298190
Reviewed-on: http://gerrit.openafs.org/11016
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>