kauth: Don't overflow cell string

The cell string within a ktc_principal is only 64 characters long.
Be careful not to overflow it.

Caught by coverity (#985829)

Change-Id: I761dfcf5eebf1bd4ef31aa5982a2593e5317257b
Reviewed-on: http://gerrit.openafs.org/9350
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>