vol: Fix gcc 4.9 warnings

gcc 4.9 complains here because the trailing 0 in these macros
has no effect, the value having already been set to NULL.
Just remove the offending 0s, nothing uses the return value
anyway, even if there were platforms where 0 != NULL.

Reviewed-on: http://gerrit.openafs.org/11176
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: D Brashear <shadow@your-file-system.com>
(cherry picked from commit c0683441a0121433d772bfb36e1e9a1c020a5dcb)

Change-Id: Ia63f56d3254b87e7d56752ec11729d82e0293792
Reviewed-on: http://gerrit.openafs.org/11204
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>