[#1568] rocketmq need enhance stability when commitlog broken

 增加非空判断，防止空指针异常,导致循环事务 check

broker/src/main/java/org/apache/rocketmq/broker/transaction/queue/TransactionalMessageBridge.java

@@ -176,8 +176,10 @@ public class TransactionalMessageBridge {
             List<ByteBuffer> messageBufferList = getMessageResult.getMessageBufferList();
             for (ByteBuffer bb : messageBufferList) {
                 MessageExt msgExt = MessageDecoder.decode(bb);
+                if(msgExt != null){
                     foundList.add(msgExt);
                 }
+            }
 
         } finally {
             getMessageResult.release();