afscp: null-terminate root.cell dir if needed in dynroot mode

instead of doing a check for something which will never be true,
and then terminating, just terminate if it's not already
null-terminated.

Change-Id: I8ad18800de22c8d10eb27fbcb2fffb2b3a5a9127
Reviewed-on: http://gerrit.openafs.org/9439
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chaskiel Grundman <cg2v@andrew.cmu.edu>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>