co_nmt_on_hb allocates on heartbeat loss but never frees
If a slave is running and heartbeat is lost, co_nmt_on_hb
(default heartbeat handler) will create an EMCY message. But there is no circumstance in which this EMCY is cleared. This means that with a setup where the heartbeat producer produces heartbeats slower than the consumer expects them, over time the system will run out of memory.