cmd: Correctly initialize cmd_OptionAsString arguments

These are coming from the stack and as such they might not be NULL.

Change-Id: Ia5c6efd08574b4de05a11dceae47021b0160395b
Reviewed-on: http://gerrit.openafs.org/10475
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>