getcapabilities avoid panic when cell is not filled

getcapabilities avoid panic when cell is not filled

for a new server, getcapabilities is not safe, as cell may not be filled.

Change-Id: I0926034c1d9e9c8c08e7ca993de1bcef9c0de8cf
Reviewed-on: http://gerrit.openafs.org/2016
Reviewed-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>