[fix] leaking the last request object after request queue is drained

src/main/java/com/android/volley/NetworkDispatcher.java

@@ -82,9 +82,11 @@ private void addTrafficStatsTag(Request<?> request) {
     @Override
     public void run() {
         Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
+        Request<?> request;
         while (true) {
             long startTimeMs = SystemClock.elapsedRealtime();
-            Request<?> request;
+            // release previous request object to avoid leaking request object when mQueue is drained.
+            request = null;
             try {
                 // Take a request from the queue.
                 request = mQueue.take();