vorbis: parse out setup headers as well

Prevents an 'Invalid packet' message. Currently mid-stream setup packets
are ignored. Theoretically, they could, based on the specification, be used to
reinitialize the stream if parameters change, but I don't expect that to be
common (and no one seems to have asked for it).

Signed-off-by: Ben Boeckel <mathstuf@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
