ext/ogg/gstoggdemux.c: When trying to find the stream length, seek back N pages instead of just one, where N is the n...

Original commit message from CVS:
* ext/ogg/gstoggdemux.c: (gst_ogg_demux_seek_before):
When trying to find the stream length, seek back N pages
instead of just one, where N is the number of streams in
the current chain.