Fix clang-analyzer false-positive on ldb_cmd.cc

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

tools/ldb_cmd.cc

@@ -1689,6 +1689,7 @@ void ReduceDBLevelsCommand::DoCommand() {
   if (exec_state_.IsFailed()) {
     return;
   }
+  assert(db_ != nullptr);
   // Compact the whole DB to put all files to the highest level.
   fprintf(stdout, "Compacting the db...\n");
   db_->CompactRange(CompactRangeOptions(), GetCfHandle(), nullptr, nullptr);