Fix valgrind warning
Fix valgrind warning

linyvxiang/dfs,imotai/bfs,baidu/bfs,imotai/bfs,yvxiang/dfs,lylei/dfs,baidu/bfs,bluebore/dfs,linyvxiang/dfs,bluebore/bfs,bluebore/bfs,imotai/bfs,baidu/bfs,yvxiang/coverage_for_bfs,myawan/bfs-1,bluebore/bfs,yvxiang/coverage_for_bfs,lylei/dfs,myawan/bfs-1,linyvxiang/dfs,bluebore/dfs,yvxiang/coverage_for_bfs,lylei/dfs,00k/ToyDFS,baidu/bfs,yvxiang/coverage_for_bfs,00k/ToyDFS,myawan/bfs-1,myawan/bfs-1,yvxiang/dfs,bluebore/dfs,yvxiang/dfs,lylei/dfs,yvxiang/dfs