[OffsetTo] Try catching nullable offsets to unbounded types

Doesn't catch all cases; if type is not fully defined at
OffsetTo time, we can't know.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1300
to the best we can do.

src/hb-open-type.hh

@@ -304,6 +304,10 @@ struct _hb_has_null<Type, true>
 template <typename Type, typename OffsetType, bool has_null=true>
 struct OffsetTo : Offset<OffsetType, has_null>
 {
+  // Make sure Type is not unbounded; works only for types that are fully defined at OffsetTo time.
+  static_assert (has_null == false ||
+		 (hb_has_null_size (Type) || !hb_has_min_size (Type)), "");
+
   HB_DELETE_COPY_ASSIGN (OffsetTo);
   OffsetTo () = default;