Only interrupt when there is no scheduler in sync_wakeup()

* When there is a scheduler, the Fiber that would be blocked has already
  been rescheduled and there is no point to interrupt something else.
  That blocked Fiber will be rescheduled as the next call to the scheduler
  (e.g., IO, sleep, other blocking sync).
* See discussion on https://github.com/ruby/ruby/commit/d01954632d

thread_sync.c

@@ -34,8 +34,10 @@ sync_wakeup(struct list_head *head, long max)
         }
 
         if (cur->th->status != THREAD_KILLED) {
+            if (cur->th->scheduler != Qnil) {
                 rb_threadptr_interrupt(cur->th);
                 cur->th->status = THREAD_RUNNABLE;
+            }
             if (--max == 0) return;
         }
     }