Don't attempt to wake tasks that aren't blocked.

It's possible for a supervised task to kill and wake its supervising
task then immediately try to wake it again if the supervising task
has joined the supervised. This is the easiest way to prevent that.

src/rt/rust_task.cpp

@@ -420,7 +420,7 @@ rust_task::notify_tasks_waiting_to_join() {
             delete waiting_task;
         } else {
             rust_task *task = waiting_task->referent();
-            if (task->dead() == false) {
+            if (task->blocked() == true) {
                 task->wakeup(this);
             }
         }