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

@@ -316,13 +316,15 @@ static inline bool apply_lookup (hb_apply_context_t *c,
    * for reverse lookup here.
    */
   for (unsigned int i = 0; i < count; /* NOP */)
-  {
-    while (c->should_mark_skip_current_glyph ())
   {
     if (unlikely (c->buffer->idx == end))
       return true;
+    while (c->should_mark_skip_current_glyph ())
+    {
       /* No lookup applied for this index */
       c->buffer->next_glyph ();
+      if (unlikely (c->buffer->idx == end))
+	return true;
     }
 
     if (lookupCount && i == lookupRecord->sequenceIndex)