Fix warning

../../src/hb-ot-layout-gsubgpos-private.hh:391:18: warning: missed loop optimization, the loop counter may overflow [-Wunsafe-loop-optimizations]

src/hb-ot-layout-gsubgpos-private.hh

@@ -388,7 +388,7 @@ struct hb_ot_apply_context_t :
     inline bool prev (void)
     {
       assert (num_items > 0);
-      while (idx >= num_items)
+      while (idx > num_items - 1)
       {
 	idx--;
 	const hb_glyph_info_t &info = c->buffer->out_info[idx];