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.

Change-Id: I344e772e9ec3d909e8b578933dd9c6c66f0a8cf6
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>