remove EbmlDummy default constructor

The whole point of EbmlDummy is to be a placeholder for elements with an unknown EBML ID.
It should not exist with a fake ID not corresponding to something read.
The current API makes some default creation mandatory but with don't have to allow
a local creation on top of that.

ebml/EbmlDummy.h

@@ -14,7 +14,6 @@ namespace libebml {
 
 class EBML_DLL_API EbmlDummy : public EbmlBinary {
   public:
-    EbmlDummy() : EbmlBinary(EbmlDummy::ClassInfos), DummyId(DummyRawId) {}
     EbmlDummy(const EbmlId & aId) : EbmlBinary(EbmlDummy::ClassInfos), DummyId(aId) {}
 
     bool IsDummy() const override {return true;}
@@ -29,7 +28,7 @@ class EBML_DLL_API EbmlDummy : public EbmlBinary {
     EbmlElement & CreateElement() const override { return Create(); }
     EbmlElement * Clone() const override { return new EbmlDummy(DummyId); }
 
-    static EbmlElement & Create() { return *(new EbmlDummy); }
+    static EbmlElement & Create() { return *(new EbmlDummy(DummyRawId)); }
     static const EbmlId & ClassId() { return DummyRawId; };
     static const EbmlCallbacks ClassInfos;