auth: Make afsconf_PutTypedKeyList idempotent

auth: Make afsconf_PutTypedKeyList idempotent

Currently, if we call afsconf_PutTypedKeyList on a key list, we set
the key list to NULL. But then if we call afsconf_PutTypedKeyList on a
NULL key list, we segfault because we try to dereference the list.

Change afsconf_PutTypedKeyList to be a noop if we give it a NULL list,
avoiding a segfault in such a situation.

Change-Id: I2c1de0c0a05ab036667031eb0e765933917826a6
Reviewed-on: https://gerrit.openafs.org/13507
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>