libacl: use initialized memory

Replace malloc with calloc to use zeroed memory when converting access
lists.

Change-Id: I17558d1737fee020772919e423c9fba37180beca
Reviewed-on: http://gerrit.openafs.org/10815
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>