firewire: sbp2: fix freeing of unallocated memory

If a target writes invalid status (typically status of a command that
already timed out), firewire-sbp2 attempts to put away an ORB that
doesn't exist.  https://bugzilla.redhat.com/show_bug.cgi?id=519772



Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>