Fix unintialized memory read in cmap subset (#1826)

src/hb-ot-cmap-table.hh

@@ -979,6 +979,14 @@ struct cmap
       if (unlikely (!format12.serialize (&c, cmap_subset_plan.format12_groups)))
 	return false;
     }
+    else
+    {
+      // FIXME: Merge this with above or, remove and tweak #final_size
+      // and rebase all the tests expectations
+      HBUINT32 empty;
+      empty = 0;
+      for (unsigned int i = 0; i < 4; ++i) c.copy (empty);
+    }
 
     c.end_serialize ();