[cff] Add memory-barrier to INDEX

Like we do for ArrayOf.

src/hb-ot-cff-common.hh

@@ -229,6 +229,7 @@ struct CFFIndex
   hb_ubytes_t operator [] (unsigned int index) const
   {
     if (unlikely (index >= count)) return hb_ubytes_t ();
+    _hb_compiler_memory_r_barrier ();
     unsigned length = length_at (index);
     if (unlikely (!length)) return hb_ubytes_t ();
     return hb_ubytes_t (data_base () + offset_at (index) - 1, length);