contrib: Fix libmatroska ebml crash

This fixes the matroska crash in at least our Windows and Linux version,
until a new libebml is released. Laurent Aimar had mostly worked around these in
[ce718de2] but FindNextElement was still vulnerable in

Tested on a matroska download with gaps. Many thanks to Laurent
(cherry picked from commit 6b7d5713)

extras/contrib/src/Makefile

@@ -948,7 +948,6 @@ libebml-$(LIBEBML_VERSION).tar.bz2:
 libebml: libebml-$(LIBEBML_VERSION).tar.bz2
 	$(EXTRACT_BZ2)
 #endif
-
 ifdef HAVE_WIN32
 	(cd $@; patch -p0 < ../Patches/libebml-svn-win32.patch)
 endif
@@ -987,7 +986,7 @@ libmatroska-$(LIBMATROSKA_VERSION).tar.bz2:
 libmatroska: libmatroska-$(LIBMATROSKA_VERSION).tar.bz2
 	$(EXTRACT_BZ2)
 #endif 
-
+	patch -p0 < ./Patches/libmatroska-kaxblock.patch
 ifdef HAVE_WIN32
 	(cd $@; patch -p0 < ../Patches/libmatroska-win32.patch)
 endif

extras/contrib/src/Patches/libmatroska-kaxblock.patch

+diff -ruN libmatroska-0.8.1/matroska/KaxBlock.h libmatroska/matroska/KaxBlock.h
+--- libmatroska-0.8.1/matroska/KaxBlock.h	2007-01-14 18:20:35.000000000 +0100
++++ libmatroska/matroska/KaxBlock.h	2008-09-23 23:05:55.000000000 +0200
+@@ -367,6 +367,7 @@
+ 	public:
+ 		KaxBlockVirtual() :ParentCluster(NULL) {Data = DataBlock; Size = countof(DataBlock);}
+ 		KaxBlockVirtual(const KaxBlockVirtual & ElementToClone);
++                ~KaxBlockVirtual() { if( Data == DataBlock ) SetBuffer( NULL, 0 ); };
+ 		static EbmlElement & Create() {return *(new KaxBlockVirtual);}
+ 		const EbmlCallbacks & Generic() const {return ClassInfos;}
+ 		static const EbmlCallbacks ClassInfos;