viced: Assert valid statistics indices

viced: Assert valid statistics indices

Make sure we are passed valid statistic index numbers, so passing in a
bad index doesn't result in writing over random memory.

Change-Id: I29ed03b59df937e6f95fba44209a03db23613a8d
Reviewed-on: http://gerrit.openafs.org/5525
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>