Initialise ptuser to NULL

Initialise ptuser to NULL

Fix fallout from d008089a79ef268bbca91d660a840f32cb416865 - sc wasn't
being initialised when it was declared, and some code paths would fail
to set it. This led to the == NULL check failing, and a new rx
connection being created with an invalid pointer as a security context.
Disaster ensued ...

Change-Id: Ibae3c72408d1145bc771bd07e573427d9f427679
Reviewed-on: http://gerrit.openafs.org/1277
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>