oggdec: Initialize stream index to -1 in ogg_packet.

The previous method of having to initialize it outside lead
to incorrect code: even if it was initialized, it usually was
only initialized once, thus a packet that could not be matched
to any stream would just be processed with the return values
from the previous call.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>