oggdemux: fix quadratic search for last page

oggdemux: fix quadratic search for last page

A crafted file with invalid pages will cause repeated searches from
earlier offsets in steps of 8500 bytes, but reading till the end of
the stream. Since we know the maximum size of an Ogg page, we can
bound the search for next page, to get a linear behavior (though
still not good enough as it will read the entire file backwards if
there's no valid page till then).