libafscp: NULL return value not pointer

When afscp_FindCallBack sets its return-by-reference value to NULL,
it should do so to the value itself, not the pointer to it.

Caught by coverity (#988419)

Reviewed-on: http://gerrit.openafs.org/9266
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 99929488511d13dab94491cac1d477062f9da741)

Change-Id: I046189684cef6c88c46455a90ab49d03da1f1cab
Reviewed-on: http://gerrit.openafs.org/9359
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>