mkv: added call to IndexAppendCluster in matroska_segment_c::Preload

It is quite obvious that we should append every Cluster as index, this
includes when doing work during Preload.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/mkv/matroska_segment.cpp

@@ -630,6 +630,7 @@ bool matroska_segment_c::Preload( )
 
             i_cluster_pos = cluster->GetElementPosition();
             ParseCluster( cluster );
+            IndexAppendCluster( cluster );
 
             ep->Down();
             /* stop pre-parsing the stream */