mkv: Ensure we don't use an EbmlDummy element for something it's not

Reported by: Zhen Zhou, NSFOCUS Security

(cherry picked from commit 89093b8c)
Signed-off-by: Hugo Beauzée-Luyssen <hugo@beauzee.fr>

modules/demux/mkv/mkv.hpp

@@ -113,7 +113,7 @@ enum
 
 #define MKVD_TIMECODESCALE 1000000
 
-#define MKV_IS_ID( el, C ) ( el != NULL && (el->operator const EbmlId&()) == (C::ClassInfos.ClassId()) )
+#define MKV_IS_ID( el, C ) ( el != NULL && (el->operator const EbmlId&()) == (C::ClassInfos.ClassId()) && !el->IsDummy() )
 #define MKV_CHECKED_PTR_DECL( name, type, src ) type * name = MKV_IS_ID(src, type) ? static_cast<type*>(src) : NULL
 #define MKV_CHECKED_PTR_DECL_CONST( name, type, src ) const type * name = MKV_IS_ID(src, type) ? static_cast<const type*>(src) : NULL