This change fixes a bug where seeking doesn't work properly for
matroska files that have the CUES element before the first cluster.
This bug was accidentally introduced a few months ago by my deferred CUES
loading patch<http://git.videolan.org/?p=ffmpeg.git;a=commit;h=31ad14c21e0735387ba8082c6e3436241f7ccfc8>
.

When the CUES element appears before the first cluster in the file, the data
is parsed and placed in matroska->index but that data is never added to the
seek index. Currently the transfer from matroska->index to the seek index
only happens when matroska_parse_cues() is called.
Matroska_parse_cues() only gets called on a seek if cues_parsing_deferred is
set. Cues_parsing_deferred only gets set if parsing the CUES requires
seeking past the first cluster. There is no code to handle the case where
CUES is before the first cluster.

This fix essentially restores the matroska->index processing that was
happening at the end of matroska_read_header() before I made my CUES
deferral change. In the case where CUES is before the first
cluster, matroska->index will have data and the seek index will be updated.
In the case where CUES is later in the file, matroska->index will be empty
and cues_parsing_deferred will be set so loading will happen later.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>