Fix the subtitles loss at MKV segment changes

This patch add the new es before deleting the old ones which causes
the default sub track to be used according to user preferences.

Fix #5625.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/mkv/virtual_segment.cpp

@@ -450,9 +450,9 @@ void virtual_segment_c::Seek( demux_t & demuxer, mtime_t i_date, mtime_t i_time_
 
         if( p_current_chapter->p_segment != p_chapter->p_segment )
         {
-            p_current_chapter->p_segment->UnSelect();
             es_out_Control( demuxer.out, ES_OUT_RESET_PCR );
             p_chapter->p_segment->Select( i_date );
+            p_current_chapter->p_segment->UnSelect();
         }
         p_current_chapter = p_chapter;