Doesn't respect conformance rule 47: events with duration = 0 don't end

[juantxorena]

As per the documentation, conformance rule 47:

VEN/VTN, EiEvent Service, oadrDistributeEvent Payload
An event with an overall duration of 0 indicates an event with no defined
end time and will remain active until explicitly canceled.

The current implementation doesn't comply, durations of 0 are taken literally, so in this point the event is marked as completed.