Check whether has been closed in FileImpl::Close
Check whether has been closed in FileImpl::Close

(#689)

lylei/dfs,linyvxiang/dfs,yvxiang/coverage_for_bfs,yvxiang/coverage_for_bfs,baidu/bfs,lylei/dfs,yvxiang/coverage_for_bfs,lylei/dfs,bluebore/bfs,yvxiang/dfs,lylei/dfs,bluebore/dfs,baidu/bfs,yvxiang/coverage_for_bfs,linyvxiang/dfs,yvxiang/dfs,baidu/bfs,yvxiang/dfs,bluebore/dfs,bluebore/bfs,baidu/bfs,yvxiang/dfs,bluebore/dfs,linyvxiang/dfs,bluebore/bfs