修复monkey测试出现的两个bug

Aria/src/main/java/com/arialyy/aria/core/queue/DownloadTaskQueue.java

@@ -106,6 +106,10 @@ public class DownloadTaskQueue implements ITaskQueue {
   }
 
   @Override public void reTryStart(Task task) {
+    if (task == null){
+      Log.w(TAG, "重试下载失败，task 为null");
+      return;
+    }
     if (!task.isDownloading()) {
       task.start();
     } else {

Aria/src/main/java/com/arialyy/aria/core/task/DownloadUtil.java

@@ -75,7 +75,7 @@ final class DownloadUtil implements IDownloadUtil {
     mDownloadEntity = entity;
     mListener = downloadListener;
     THREAD_NUM = threadNum;
-    mFixedThreadPool = Executors.newFixedThreadPool(THREAD_NUM);
+    mFixedThreadPool = Executors.newFixedThreadPool(Integer.MAX_VALUE);
   }
 
   public IDownloadListener getListener() {
@@ -292,7 +292,7 @@ final class DownloadUtil implements IDownloadUtil {
             for (int l : recordL) {
               if (l == -1) continue;
               Runnable task = mTask.get(l);
-              if (task != null) {
+              if (task != null && !mFixedThreadPool.isShutdown()) {
                 mFixedThreadPool.execute(task);
               }
             }