decodebin: For adaptive streaming, ensure to put the buffering multiqueue after a...

decodebin: For adaptive streaming, ensure to put the buffering multiqueue after a parser or demuxer

There are cases when there is no demuxer involved that could do the
buffering, e.g. HLS with raw MP3 or AAC. In this case we want to place
the buffering multiqueue after the parser.

Before this change, we've considered the first element after the
adaptive streaming demuxer as a parser. This is not always true, e.g.
id3demux. Instead we now wait until we actually have a parser (or
decoder).

Fixes playback on such HLS streams.