fix crash in hb_map_t::clear()

in case called immediately after init()

src/hb-map.hh

@@ -170,7 +170,8 @@ struct hb_map_t
 
   void clear ()
   {
-    memset (items, 0xFF, ((size_t) mask + 1) * sizeof (item_t));
+    if (items)
+      memset (items, 0xFF, ((size_t) mask + 1) * sizeof (item_t));
     population = occupancy = 0;
   }