auth: check if argument of afsconf_Close* is null

auth: check if argument of afsconf_Close* is null

Currently, we do not check if the argument of afsconf_Close /
afsconf_CloseInternal is equal to null. In order to avoid a possible
segmentation fault, add the checks.

Change-Id: I45635ad2d735505637072867edb7ff17da3c671a
Reviewed-on: https://gerrit.openafs.org/13352
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>