mcas: Make sure 'padding' is null-terminated

mcas: Make sure 'padding' is null-terminated

With 'padding' explicitly filled with all spaces string copy operations
may result in unexpected values.  Padding is extended by 1 and null
terminated to avoid unexpected behavior.
(via cppcheck)

Change-Id: I8a9845ae87002018705ad23c2b089c8ef571b7bc
Reviewed-on: https://gerrit.openafs.org/13164
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>