Summary:
Recent changes trigger clang analyze warning. Fix it.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5870

Test Plan: "USE_CLANG=1 TEST_TMPDIR=/dev/shm/rocksdb OPT=-g make -j60 analyze" and make sure it passes.

Differential Revision: D17682533

fbshipit-source-id: 02716f2a24572550a22db4bbe9b54d4872dfae32