Summary:
clang-analyzer complaint about db_ being nullptr, but it couldn't be because it checks exec_stats before proceed. Add an assert to get around the false-positive.
Test Plan
`make analyze`
Closes https://github.com/facebook/rocksdb/pull/3236
Differential Revision: D6505417
Pulled By: yiwu-arbug
fbshipit-source-id: e5b65764ea994dd9e4bab3e697b97dc70dc22cab