Mozilla Bug 618592 - freeze on typekit

Fix apply_lookup() for zero-input broken fonts.

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

@@ -230,13 +230,13 @@ static inline bool apply_lookup (hb_apply_context_t *c,
 				 apply_lookup_func_t apply_func)
 {
   unsigned int end = MIN (c->buffer->len, c->buffer->i + c->context_length);
-  if (unlikely (c->buffer->i + count > end))
+  if (unlikely (count == 0 || c->buffer->i + count > end))
     return false;
 
   /* TODO We don't support lookupRecord arrays that are not increasing:
    *      Should be easy for in_place ones at least. */
 
-  /* Note: If sublookup is reverse, i will underflow after the first loop
+  /* Note: If sublookup is reverse, it will underflow after the first loop
    * and we jump out of it.  Not entirely disastrous.  So we don't check
    * for reverse lookup here.
    */