Fix hb-ot-shape-normalize with empty buffer

Part of https://github.com/behdad/harfbuzz/issues/136

src/hb-ot-shape-normalize.cc

@@ -290,6 +290,8 @@ _hb_ot_shape_normalize (const hb_ot_shape_plan_t *plan,
 			hb_buffer_t *buffer,
 			hb_font_t *font)
 {
+  if (unlikely (!buffer->len)) return;
+
   _hb_buffer_assert_unicode_vars (buffer);
 
   hb_ot_shape_normalization_mode_t mode = plan->shaper->normalization_preference;