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)

Change-Id: I6b227465d7aa945d871b2db1cc4d1357cae493d3
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>