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.

Change-Id: Ic9a79d51419726c0c823a9228c21c13dea918dc8
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>