libadmin: Clear structures according to their size.

memset(a, 0, sizeof(a)) is rarely correct, unless a is an error. Use the
size of the destination structure, rather than the size of a pointer to it
when deciding how much memory to clear.

Caught by clang's new error messages

Reviewed-on: http://gerrit.openafs.org/7076
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 34afb3e6b903fc1659b1c869fa2fb4226038e076)

Change-Id: I06912a70af9f3cd54f4a5368e996a7308e687c20
Reviewed-on: http://gerrit.openafs.org/10243
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>