lavf/oggdec: check for begin-of-stream flag in case of chained streams.

Fix Ticket #1617, revealing a regression I introduced in 8f3eebd.

We need to make sure no stream is added in between Ogg context save and
restore operations (because it would likely lead to a mismatch between
ogg->nstreams and AVFormatContext->nb_streams after the restore op).
This is the reason the ogg->state check is added in ogg_new_stream().
Before this patch, checking for ogg->headers was preventing this:
ogg->headers is always set before any ogg save/restore (though, it was
also preventing from creating the stream when necessary).
