kauth: Handle calls to ka_log with no principal

If ka_log is called without a principal string, then the resulting
buffer will be garbage, as we don't start with a string for strlcat
to append to.

Caught by coverity (#985959)

Change-Id: I928b2807c093ac3f71a28150a117fd7b7eb29b05
Reviewed-on: http://gerrit.openafs.org/9396
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>