cleaner: Fix null pointer exception in 'hsms attached to pool' command

Addresses a null pointer exception in the cleaner.

Ticket: http://rt.dcache.org/Ticket/Display.html?id=7769
Target: trunk
Request: 2.6
Request: 2.2
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/5493/
Merge pull request #11 from gbehrmann/fix/2.2/rb5493

cleaner: Fix null pointer exception in 'hsms attached to pool' command