Initialize variable to avoid warning
Initialize variable to avoid warning

Summary:
RocksDB debug version failed to build under gcc-4.8.1 on sandcastle with the following error:

```
db/db_compaction_filter_test.cc:570:33: error: â€˜snapshotâ€™ may be used uninitialized in this function [-Werror=maybe-uninitialized]
```

Test Plan: make db_compaction_filter_test && ./db_compaction_filter_test

Reviewers: rven, anthony, yhchiang, aekmekji, igor, sdong

Reviewed By: igor, sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D46725

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