aklog: Fix improper use of readlink

aklog: Fix improper use of readlink

readlink doesn't NUL terminate its return string, so it is up to
us to do so.

Caught by coverity (#985739)

Reviewed-on: http://gerrit.openafs.org/9347
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 2fac53522e7ef5b3a376e191bffdc1f6784e6995)

Change-Id: I9d47a6a7cbc86fba3f68f7e47c5d7a0fb924781f
Reviewed-on: http://gerrit.openafs.org/11035
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>