[ENOMEM] don't perform set process operations if the other set is in an error state.

Running a process while the other set is in an error state can potentially corrupt this sets map map (for example by overwritting all of the major values with 0).

src/hb-set.hh

@@ -544,6 +544,7 @@ struct hb_set_t
   void process (const Op& op, const hb_set_t *other)
   {
     if (unlikely (!successful)) return;
+    if (unlikely (!other->successful)) return;
 
     dirty ();