Commit 78a8771f attempted to reset the ogg_stream_state in
find_last_frame(), but it tries to do so using a non-existent page
and then leaks memory (due to using ogg_stream_init() instead of
ogg_stream_reset()). Its caller, Ogg_GetLastPacket(), then tries
to restore the contents of the current packet, but its call to
ogg_stream_pageseek() will immediately fail, leaving the dangling
packet pointers untouched.

Instead, just back up the whole stream state and restore it on
exit. This fixes both problems.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>