afs: do not leak stale data in buffers

afs: do not leak stale data in buffers

Similar to the previous commit, zero out the buffer when fetching
a new slot, to avoid the possibility of leaving stale data in
a reused buffer.

We are not supposed to write such stale data back to a fileserver,
but this is an extra precaution in case of bugs elsewhere -- memset
is not as expensive as it was in the 1980s.

Reviewed-on: https://gerrit.openafs.org/12459
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Tested-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit a26c5054ee501ec65db3104f6a6a0fef634d9ea7)

Change-Id: Id60559ed84581e2f6a50cd4313f64780b8a0bafd
Reviewed-on: https://gerrit.openafs.org/12464
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Tested-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>