qtdemux: Fix segmentation fault in check_update_duration ()

In the gst_qtdemux_stream_flush_segments_data () method, all segments
related variables should be set as default.

When check_update_duration () method was called with following condition,
demux will try to do invalid memory access
* dummy segment had been created previously.
  This will allocate memory for stream->segments with setting dummy_segment = TRUE
* gst_qtdemux_stream_flush_segments_data () has been called by some reason.
  This will free stream->segments without setting dummy_segment = FALSE

Note that, check_update_duration () will try to update duration of stream->segments
if dummy_segment was set to TRUE