[serialize] Check offset base is within (possibly end of) object

src/hb-serialize.hh

@@ -295,6 +295,7 @@ struct hb_serialize_context_t
       {
         const object_t::link_t &link = *link_it;
 	const object_t &child = *packed[link.objidx];
+	assert (link.bias <= parent.tail - parent.head);
 	unsigned offset = (child.head - parent.head) - link.bias;
 
 	if (link.is_wide)