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

Change-Id: Ib011da471465dae056d47f5bbaad5a27b5c85966
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>