Don't delete when the file is /dev/null

[zhaixiaojuan]

What problem does this PR solve?

Issue Number: #2345

Problem Summary:
由于iobuf_unittest.cpp文件中append_store_append_cut测试用例删除/dev/null，导致在for循环中在执行open时重新创建了一个文件类型的/dev/null，从而引发系统问题

What is changed and the side effects?

Changed: 当检测到是/dev/null, 不对其进行删除

Side effects:

• Performance effects(性能影响):
  无

• Breaking backward compatibility(向后兼容性):
  无

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