[serialize] Don't insert empty object into tree

src/hb-serialize.hh

@@ -160,6 +160,9 @@ struct hb_serialize_context_t
     memmove (tail, obj.head, len);
     head = obj.head;
 
+    if (!len)
+      return 0;
+
     obj.head = tail;
     obj.tail = tail + len;
 
@@ -304,7 +307,7 @@ struct hb_serialize_context_t
   hb_vector_t<object_t> packed;
 
   /* Map view of packed objects. */
-  hb_hashmap_t<const object_t *, objidx_t> packed_map;
+  hb_hashmap_t<const object_t *, objidx_t, nullptr, 0> packed_map;
 };