mkv: Introduced MKV_CHECKED_PTR_DECL

`MKV_IS_ID` is a macro to check whether the object hiding behind a pointer
has a certain dynamic type and it is being used all over the module.

There are however a lot of cases that is followed by a static_cast to the
appropriate type; `MKV_CHECKED_PTR_DECL` has been introduced to shorten
code that has such usage.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/mkv/mkv.hpp

@@ -112,6 +112,7 @@ enum
 #define MKVD_TIMECODESCALE 1000000
 
 #define MKV_IS_ID( el, C ) ( el != NULL && typeid( *el ) == typeid( C ) )
+#define MKV_CHECKED_PTR_DECL( name, type, src ) type * name = MKV_IS_ID(src, type) ? static_cast<type*>(src) : NULL
 
 
 using namespace LIBMATROSKA_NAMESPACE;