Reinstante DEFINE_NULL_DATA

Seems like I messed up; buffer overrun got reported.

src/hb-open-type-private.hh

@@ -685,6 +685,7 @@ struct Tag : HBUINT32
   public:
   DEFINE_SIZE_STATIC (4);
 };
+DEFINE_NULL_DATA (OT, Tag, "    ");
 
 /* Glyph index number, same as uint16 (length = 16 bits) */
 typedef HBUINT16 GlyphID;
@@ -696,6 +697,7 @@ typedef HBUINT16 NameID;
 struct Index : HBUINT16 {
   static const unsigned int NOT_FOUND_INDEX = 0xFFFFu;
 };
+DEFINE_NULL_DATA (OT, Index, "\xff\xff");
 
 /* Offset, Null offset = 0 */
 template <typename Type>

src/hb-ot-layout-common-private.hh

@@ -165,6 +165,7 @@ struct RangeRecord
   public:
   DEFINE_SIZE_STATIC (6);
 };
+DEFINE_NULL_DATA (OT, RangeRecord, "\000\001");
 
 
 struct IndexArray : ArrayOf<Index>
@@ -224,6 +225,7 @@ struct LangSys
   public:
   DEFINE_SIZE_ARRAY (6, featureIndex);
 };
+DEFINE_NULL_DATA (OT, LangSys, "\0\0\xFF\xFF");
 
 
 struct Script
@@ -245,16 +247,7 @@ struct Script
   { return langSys.find_index (tag, index); }
 
   inline bool has_default_lang_sys (void) const { return defaultLangSys != 0; }
-  inline const LangSys& get_default_lang_sys (void) const
-  {
-    if (!defaultLangSys)
-    {
-      /* This is the ONLY place where our null data is not all zeros.
-       * So, return special data instead of using the null pool. */
-      return *reinterpret_cast<const LangSys *> ("\0\0\xFF\xFF");
-    }
-    return this+defaultLangSys;
-  }
+  inline const LangSys& get_default_lang_sys (void) const { return this+defaultLangSys; }
 
   inline bool sanitize (hb_sanitize_context_t *c,
 			const Record<Script>::sanitize_closure_t * = nullptr) const

src/hb-private.hh

@@ -1117,5 +1117,18 @@ static inline const Type& Null (void) {
 }
 #define Null(Type) Null<Type>()
 
+/* Specializaiton for arbitrary-content arbitrary-sized Null objects. */
+#define DEFINE_NULL_DATA(Namespace, Type, data) \
+} /* Close namespace. */ \
+static const char _Null##Type[sizeof (Namespace::Type) + 1] = data; /* +1 is for nul-termination in data */ \
+template <> \
+/*static*/ inline const Namespace::Type& Null<Namespace::Type> (void) { \
+  return *reinterpret_cast<const Namespace::Type *> (_Null##Type); \
+} \
+namespace Namespace { \
+/* The following line really exists such that we end in a place needing semicolon */ \
+static_assert (Namespace::Type::min_size + 1 <= sizeof (_Null##Type), "Null pool too small.  Enlarge.")
+
+
 
 #endif /* HB_PRIVATE_HH */