fix crash in hb_map_t::clear()

in case called immediately after init()
diff --git a/src/hb-map.hh b/src/hb-map.hh
index 02d5406..fd5c6d8 100644
--- a/src/hb-map.hh
+++ b/src/hb-map.hh
@@ -170,7 +170,8 @@
 
   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;
   }