ext/ogg/gstoggdemux.c: Refix oggdemux, we only have a problem if we failed to find a chain and we are not EOF.

Original commit message from CVS:
* ext/ogg/gstoggdemux.c: (gst_ogg_demux_activate_chain),
(gst_ogg_demux_read_chain):
Refix oggdemux, we only have a problem if we failed to find a chain and
we are not EOF.