For FLAC, Speex, Opus and VP8 the Ogg muxer allocates two buffers
for building the headers: The first for extradata in an Ogg-specific
format and the second contains a Vorbiscomment. These buffers are
reachable via pointers in the corresponding AVStream's priv_data.

If an error happens during building the headers, the AVStream's
priv_data would be freed. This is pointless in general as it would be
freed generically anyway, but here it is actively harmful: If the second
of the aforementioned allocations fails, the first buffer would leak
upon freeing priv_data.

This commit stops freeing priv_data manually, which allows the muxer to
properly clean up in the deinit function.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>