butc: Init volheader before using it for hton

When converting a volume header from host to network order, make
sure that any unused fields in the structure are zero'd, so we don't
end up filling them with stack garbage in the network version of
the structure.

Caught by coverity (#985956)

Reviewed-on: http://gerrit.openafs.org/9548
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 21166744bf15f13bc04c25d68a28fe803a960b0a)

Change-Id: I7156cc51f52a8c9911d6ce087ba0a6b1cc0d9a9e
Reviewed-on: http://gerrit.openafs.org/11065
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>