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.

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>
(cherry picked from commit ceac74a6baad79155adaa692639b1d9f403525b6)

Change-Id: I14d3de25bca5b1ba1878048aac1973518f8c2359
Reviewed-on: http://gerrit.openafs.org/9583
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>