Fixes ticket #6804. All of the ogg header and packet parsers may
return standard AVERROR codes; these return values should not be
treated as success.

Additionally changes oggparsevorbis, to not give up too early
with certain types of poorly muxed files.
Signed-off-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>