[map] Fix bad memory access if hb_map.fini() was called twice.

src/hb-map.hh

@@ -131,11 +131,13 @@ struct hb_hashmap_t
   }
   void fini_shallow ()
   {
-    unsigned size = mask + 1;
-    for (unsigned i = 0; i < size; i++)
-      items[i].~item_t ();
-    hb_free (items);
-    items = nullptr;
+    if (unlikely (!items)) {
+      unsigned size = mask + 1;
+      for (unsigned i = 0; i < size; i++)
+        items[i].~item_t ();
+      hb_free (items);
+      items = nullptr;
+    }
     population = occupancy = 0;
   }
   void fini ()

src/test-map.cc

@@ -78,6 +78,14 @@ main (int argc, char **argv)
     assert (v.get_population () == 2);
   }
 
+  /* Test call fini() twice. */
+  {
+    hb_map_t s;
+    for (int i = 0; i < 16; i++)
+      s.set(i, i+1);
+    s.fini();
+  }
+
   /* Test initializing from iterator. */
   {
     hb_map_t s;