aklog: strlen(NULL) doesn't work

aklog: strlen(NULL) doesn't work

strlen(filepath) when !filepath isnt going to work very well. i believe
this to be the intent of the author of the original patch.

Reviewed-on: http://gerrit.openafs.org/5328
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit c3e82ee98bf66058636f11d7a98d3bebe3bac955)

Change-Id: I89911d2da314059db633c00c69c9c9ec2050bb86
Reviewed-on: http://gerrit.openafs.org/6311
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>