adaptivedemux: doesn't answer duration queries for live streams

Submitted by mariuszb

Link to original bug (#753879)

Description

For duration queries on live streams adaptivedemux ignores the query. The problem then is that the query is answered by qtdemux downstream for with the values valid for the currently passing fragment.

I think adaptivedemux should be the final authority on stream duration and should return valid duration with GST_CLOCK_TIME_NONE set as a value for live streams.