If xnregistry_enter in mq_open fails, the new entry will not be added to
cobalt_mqq. But the cleanup path taken afterwards will nevertheless try
to remove it from there. That will cause a crash because the mq->link is
uninitialized. Fix that by initializing it as list head so that list
removal becomes harmless, even when the entry is not on any list..

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>