Fix more possible buffer overruns

I have this function, but can't clean up it to my satisfaction.

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 f1d03dc..13386c2 100644
--- a/src/hb-ot-layout-gsubgpos-private.hh
+++ b/src/hb-ot-layout-gsubgpos-private.hh
@@ -317,12 +317,14 @@
    */
   for (unsigned int i = 0; i < count; /* NOP */)
   {
+    if (unlikely (c->buffer->idx == end))
+      return true;
     while (c->should_mark_skip_current_glyph ())
     {
-      if (unlikely (c->buffer->idx == end))
-	return true;
       /* No lookup applied for this index */
       c->buffer->next_glyph ();
+      if (unlikely (c->buffer->idx == end))
+	return true;
     }
 
     if (lookupCount && i == lookupRecord->sequenceIndex)