gst/matroska/matroska-demux.c: Fix setting the discont flag on the first buffer pushed downstream for formats with pr...

Original commit message from CVS:
* gst/matroska/matroska-demux.c:
Fix setting the discont flag on the first buffer
pushed downstream for formats with private codec
data that needs to be deserialised into buffers
(such as vorbis and FLAC when in a matroska container).