Unix CM: Don't blow up if we have non-rxkad tokens

Unix CM: Don't blow up if we have non-rxkad tokens

The legacy GetToken compatibility code assumed that we would always
have at least one rxkad token for a cell, and segfaulted if we didn't.

Just return as if we have no tokens if a cell doesn't have an rxkad
token.

Change-Id: Ica64882c1a64cf140ad6c277c70fbfb747ea6235
Reviewed-on: http://gerrit.openafs.org/3614
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>