kauth: realm can't be NULL when calling CellToRealm

Despite what the static analyzer thinks, ka_CellToRealm can't be
called with a NULL realm. Just add an assert to close down that
code path, and stop it from producing an error.

Change-Id: Ifc8cf7e5dca5ae29eaaa91c5b238a97197d89ee3
Reviewed-on: http://gerrit.openafs.org/9174
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>