Windows: FSCTL_SET_REPARSE_POINT NULL ptr dereference

Avoid a potential NULL pointer dereference if the ParentObjectInfo
object cannot be found.  If the Btree lookup fails, do not call
AFSCreateSymlink() and do not decrement the ObjectInfo refcount.
Doing so will result in a BSOD.

Change-Id: Ibd3e4ebb343f6c3cff8bf1cb160e42938d0f906c
Reviewed-on: http://gerrit.openafs.org/9643
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Peter Scott <pscott@kerneldrivers.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>