libadmin: Don't free garbage

Make sure that we initialise the nbulkentries structure to 0 before
we start work, so that if the failure handler is called, it doesn't
try to free garbage.

Caught by coverity (#985980)

Change-Id: If55faadd44f7bbc80abbdcd7842603d586db7581
Reviewed-on: http://gerrit.openafs.org/9317
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>