Scheduler: Handle weird cases when the task we're iterating on is null

kernel/src/thread/Scheduler.cpp

@@ -40,6 +40,7 @@ template <typename Callback> void sched_for_each_task(Callback callback)
     Task* task = base_task;
     if (!task) return;
     do {
+        if (!task) break;
         bool will_continue = callback(task);
         if (!will_continue) break;
         task = task->next_task;