Thread.Farm: typo in call to cond->wait()

lib/modules/Thread.pmod

@@ -607,7 +607,7 @@ optional class Farm
         threads += ({ Handler() });
         free_threads += ({ threads[-1] });
       } else {
-        ft_cond->wait(mutex);
+        ft_cond->wait(lock);
       }
     }
     object(Handler) t = free_threads[0];