Additional fix to last-page handling, this time in initial bisection;

Additional fix to last-page handling, this time in initial bisection;
the code had gotten too cavalier about directly munging the vf->offset
file pointer as well as using it as an implicit argument to
_get_prev_page and _get_prev_page_serial.  The fact it was being used
as an argument and a side effect caused a necessary seek to get missed
when the last page of a link was non-vorbis.

Fix: Clean up the overloading, and be explicit about where we're
beginning prev_page reads.

svn path=/trunk/vorbis/; revision=19165