fix db_bench filluniquerandom key count assertion

Summary:
It failed every time. I guess people usually ran with assertions disabled.
Closes https://github.com/facebook/rocksdb/pull/3422

Differential Revision: D6822984

Pulled By: ajkr

fbshipit-source-id: 2e90db75618b26ac1c46ddfa9e03c095c7bf16e3

tools/db_bench_tool.cc

@@ -3504,7 +3504,7 @@ void VerifyDBFromDB(std::string& truth_db_name) {
         case RANDOM:
           return rand_->Next() % num_;
         case UNIQUE_RANDOM:
-          assert(next_ + 1 < num_);
+          assert(next_ < num_);
           return values_[next_++];
       }
       assert(false);