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.

Change-Id: I363310665313f7bbcca783a4d9be87aaa7b0a8fe
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>