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.

Reviewed-on: http://gerrit.openafs.org/11627
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 34e495d69a8831c57cac2ccf18898e63f02c7745)

Change-Id: Ia8250ed6e699d2ed8e2b5de7481c54ae65b0718c
Reviewed-on: http://gerrit.openafs.org/11826
Reviewed-by: Daria Phoebe Brashear <shadow@your-file-system.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>