butc: do not use stack garbage for secobj

The butc only ever handles connections over rxnull anyway, so just
specify an array of security objects of length one.

Change-Id: Ia073157488ac6b887f972448074ad1615422e26e
Reviewed-on: http://gerrit.openafs.org/9140
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>