In closef(), remove the assumption that there is a thread associated

In closef(), remove the assumption that there is a thread associated
with the file descriptor.  When a file descriptor is closed as a result
of garbage collecting a UNIX domain socket, the file descriptor will
not have any associated thread, so the logic to identify advisory locks
held by that thread is not appropriate.  Check the thread for NULL to
avoid this scenario.  Expand an existing comment to say a bit more about
this.

MFC after:	1 week