fix garbage counters in supergroup prdb_check

This is a backport of one hunk of commit
9efc255ab8de7ad90c7d599fea7bd1956bfd48c2, where a calloc fixes an
incorrectly sized memset. The original memset clears the size of the
pointer, not the sizeof the structure.

Change-Id: I40250e0346345fe9b4b43d14e76ea5fb23c8c04b
Reviewed-on: http://gerrit.openafs.org/9701
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>