oggdemux: fix seeking with negative rate with skeleton

oggdemux: fix seeking with negative rate with skeleton

Files with a skeleton, or other files with a stream that ends before the end of
the chain would start playing from the end of the chain when trying to seek with
a negative rate at a position between the end of any stream and the end of the
chain.

This is due to the loop in _do_seek() assuming that pages will be encountered
for all streams shortly after the place where we want to seek, as found by
do_binary_search().

In the first iteration of the loop, stream ends are now checked against the
time of the current page.