fix use_of_uninitialized_value in message_loop_task_queues.cc (flutter#55520)

This issue was found with memory sanitizer.

    Commit f2f09b6 introduced a change that leads to use-after-free condition.

    In function MessageLoopTaskQueues::GetNextTaskToRun:

        1) Call is made to PeekNextTaskUnlocked(queue_id);. Returned value contains a reference to to an object of const DelayedTask& taken from an std::queue container as returned by primary_task_queue_.top().
        2) Variable TaskSource::TopTask top now contains a reference to this object.
        3) Function queue_entries_.at(top.task_queue_id)->task_source->PopTask(...) which in turn calls pop() method on std::queue.
        4) Object of type DelayedTask on top of the queue gets deleted.
        5) top.task.GetTaskSourceGrade() is called later with top.task refering to an already deleted object.

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Author: art-snake

fml/message_loop_task_queues.cc

@@ -132,9 +132,8 @@ fml::closure MessageLoopTaskQueues::GetNextTaskToRun(TaskQueueId queue_id,
     return nullptr;
   }
   fml::closure invocation = top.task.GetTask();
-  queue_entries_.at(top.task_queue_id)
-      ->task_source->PopTask(top.task.GetTaskSourceGrade());
   const auto task_source_grade = top.task.GetTaskSourceGrade();
+  queue_entries_.at(top.task_queue_id)->task_source->PopTask(task_source_grade);
   tls_task_source_grade.reset(new TaskSourceGradeHolder{task_source_grade});
   return invocation;
 }