Improve exception safety with smart pointers

Would you like to wrap any pointers with the class template “[std::unique_ptr](https://en.wikipedia.org/wiki/Smart_pointer#unique_ptr "Description for the usage of smart pointers")”?

Update candidates:
- [EnvMirrorTest](https://github.com/facebook/rocksdb/blob/fdf882ded218344c136c97daf76dfb59e4bc155f/utilities/env_mirror_test.cc#L22)
- [LogReaderContainer](https://github.com/facebook/rocksdb/blob/fdf882ded218344c136c97daf76dfb59e4bc155f/db/db_impl/db_impl_secondary.h#L21)