AsyncPool: assert() that the unstacked task is actually an AsyncTask

it's possible that it might not be if the workers were accessed directly, but that shouldn't be possible.
This also silences a PHPStan warning on level 2.

src/pocketmine/scheduler/AsyncPool.php

@@ -263,6 +263,7 @@ class AsyncPool{
 			while(($task = $worker->unstack()) !== null){
 				//cancelRun() is not strictly necessary here, but it might be used to inform plugins of the task state
 				//(i.e. it never executed).
+				assert($task instanceof AsyncTask);
 				$task->cancelRun();
 				$this->removeTask($task, true);
 			}