SegmentList nodes are allowed into Period, AdaptationSet or Representation nodes
and there is at most 1 element, so no need to keep a list;
Period nodes cannot have any Represention elements, as AdaptationSet nodes are mandatory;
this breaks compatibility with some legacy DASH test sequences.
gstmpdparser.c: In function ‘gst_mpdparser_get_list_and_nb_of_audio_language’:
gstmpdparser.c:2891: warning: ‘return’ with no value, in function returning non-void
