Windows: avoid null cellp dereference during dump

When performing a memory dump if cm_ucell_t.cellp is NULL, do
not attempt to print the cell name.

Change-Id: I146b90b5424b93d0c52a758b33fcff1bcf441e77
Reviewed-on: http://gerrit.openafs.org/9582
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>