kauth: Catch ka_KeyCheckSum failures

If ka_KeyCheckSum fails, typically because the key being used is bad,
catch the failure rather than just silently ignoring it.

Caught by clang-analyzer

Change-Id: Id51174fdf94356db10961690808670da372bc60e
Reviewed-on: http://gerrit.openafs.org/9179
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>