volscan: avoid printing null mount-point cellname

volscan: avoid printing null mount-point cellname

The 'mcell' field is null when there is no cell set in the mount point,
so do not try to print it.

Reviewed-on: http://gerrit.openafs.org/10226
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: D Brashear <shadow@your-file-system.com>
(cherry picked from commit db2ed13359ce8e3cd6a4981f8ce8e10ba61f6463)

Change-Id: I96ba7dec59cc6e0fc5663e8e037d76d1006905a7
Reviewed-on: http://gerrit.openafs.org/11275
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: Stephan Wiesand <stephan.wiesand@desy.de>