Currently (since the data_offset fix) the ogg demuxer assumes that
after the first non-header packets in any stream no more header packets
will follow.
This is not guaranteed, so change the code back again to wait until it
has finished the headers for all streams before returning from ogg_get_headers.

This fixes issue 2428.
Signed-off-by: Mans Rullgard <mans@mansr.com>
(cherry picked from commit 6bd69e6a)