Added an assert in mkv.

(cherry picked from commit 2784bc2a)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/mkv/Ebml_parser.cpp

@@ -50,6 +50,7 @@ EbmlParser::~EbmlParser( void )
 {
     if( !mi_level )
     {
+        assert( !mb_keep );
         delete m_el[1];
         return;
     }