asetkey: Handle unknown key types

If the user tries to create a key which is not rxkad, error out
rather than attempt to add a random blob of memory to the KeyFile.

Caught by clang-analyzer

Change-Id: Id2803fe29153f01ab352747caed31b0d0cb4f573
Reviewed-on: http://gerrit.openafs.org/9160
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>