Handle backupDate of zero

Handle backupDate of zero

In older versions of OpenAFS (prior to 2001), the backupDate was
never set.  Try to provide somewhat more reasonable behavior in
this case, by using a different date in that case.

Change-Id: Id74ce003c6a2317b06e78ba64d6891229c16421a
Reviewed-on: http://gerrit.openafs.org/11627
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>