Fix mark skipping regression

Ouch!

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

diff --git a/src/hb-ot-layout-gsubgpos-private.hh b/src/hb-ot-layout-gsubgpos-private.hh
index f22306f..8dbfc16 100644
--- a/src/hb-ot-layout-gsubgpos-private.hh
+++ b/src/hb-ot-layout-gsubgpos-private.hh
@@ -88,9 +88,9 @@
     {
       do
       {
-	idx++;
 	if (has_no_chance ())
 	  return false;
+	idx++;
       } while (_hb_ot_layout_skip_mark (c->face, &c->buffer->info[idx], lookup_props, property_out));
       num_items--;
       return true;