ebml: fix a potential crash

--- ebml/src/EbmlBinary.cpp	2018-02-09 13:51:37.752623200 +0100
+++ ebml/src/EbmlBinary.cpp.null_cmp	2018-02-09 13:50:41.409429100 +0100
@@ -100,7 +100,7 @@ filepos_t EbmlBinary::ReadData(IOCallbac
 
 bool EbmlBinary::operator==(const EbmlBinary & ElementToCompare) const
 {
-  return ((GetSize() == ElementToCompare.GetSize()) && !memcmp(Data, ElementToCompare.Data, GetSize()));
+  return ((GetSize() == ElementToCompare.GetSize()) && (GetSize() == 0 || !memcmp(Data, ElementToCompare.Data, GetSize())));
 }
 
 END_LIBEBML_NAMESPACE