libavformat/oggdec: Free stream private when header parsing fails.

Leaking this private structure opens up the possibility that it may
be re-used when parsing later packets in the stream. This is
problematic if the later packets are not the same codec type (e.g.
private allocated during Vorbis parsing, but later packets are Opus
and the private is assumed to be the oggopus_private type in
opus_header()).

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
