MKV: fix meta regression

(cherry picked from commit 57d06767)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/mkv/matroska_segment.cpp

@@ -371,6 +371,7 @@ void matroska_segment_c::LoadTags( KaxTags *tags )
  *****************************************************************************/
 void matroska_segment_c::InformationCreate( )
 {
+    if( !sys.meta )
         sys.meta = vlc_meta_New();
 
     if( psz_title )