Erasure Coding: Fix the NullPointerException when deleting file

In HDFS, when removing some file, NN will also remove all its blocks from BlocksMap, and send DNA_INVALIDATE (invalidate blocks) commands to datanodes. After datanodes successfully delete the block replicas, will report DELETED_BLOCK to NameNode.

snippet code logic in BlockManager#processIncrementalBlockReport is as following

case DELETED_BLOCK:
        removeStoredBlock(storageInfo, getStoredBlock(rdbi.getBlock()), node);
        ...

private void removeStoredBlock(DatanodeStorageInfo storageInfo, Block block,
      DatanodeDescriptor node) {
    if (shouldPostponeBlocksFromFuture &&
        namesystem.isGenStampInFuture(block)) {
      queueReportedBlock(storageInfo, block, null,
          QUEUE_REASON_FUTURE_GENSTAMP);
      return;
    }
    removeStoredBlock(getStoredBlock(block), node);
  }

In EC branch, we add getStoredBlock. There is NullPointerException when handling DELETED_BLOCK of incrementalBlockReport from DataNode after delete a file, since the block is already removed, we need to check.