Windows: Protect against cm_GetVolServerList failures

In cm_Analyze, if cm_GetVolServerList() fails volServerpp will be
NULL which will trigger an exception if passed to either
cm_SetServerBusyStatus or cm_ResetServerBusyStatus.

Change-Id: I75b4b855b8c3ccfc014532b0c2eb3135807647ef
Reviewed-on: http://gerrit.openafs.org/9960
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>