Original commit message from CVS:
* ext/ogg/gstoggdemux.c: (gst_ogg_pad_typefind),
(gst_ogg_demux_perform_seek), (gst_ogg_demux_sink_activate):
* ext/vorbis/vorbisdec.c: (vorbis_dec_convert),
(vorbis_dec_src_query), (vorbis_dec_src_event),
(vorbis_dec_sink_event), (vorbis_handle_comment_packet),
(vorbis_handle_type_packet), (vorbis_handle_header_packet),
(copy_samples), (vorbis_handle_data_packet), (vorbis_dec_chain):
Don't crap out when seeking back to position 0.