auth: Handle empty lists of keys

auth: Handle empty lists of keys

Handle the case where a list of keys may be empty - don't attempt
to calloc a 0 length array, and have Put handle the case where the
array being freed has no elements.

Caught by clang-analyzer

Change-Id: I665035cb00a8da411710a71a17ae66545702127f
Reviewed-on: http://gerrit.openafs.org/7098
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>