vol: fix vutil format-truncation nit

vol: fix vutil format-truncation nit

We need one more byte for the trailing NUL.

Reviewed-on: https://gerrit.openafs.org/13462
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit bfe912ede6f452d10cfbd5fd549f44ee027acb1b)

Change-Id: I1843bd9cb3392f721068c079b69fca65f6d1a181
Reviewed-on: https://gerrit.openafs.org/13731
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>