Fix SIGSEGV when not using cache

db/db_bench.cc

@@ -1423,9 +1423,11 @@ class Benchmark {
                   [](ColumnFamilyHandle* cfh) { delete cfh; });
     delete db_.db;
     delete prefix_extractor_;
+    if (cache_.get() != nullptr) {
       // this will leak, but we're shutting down so nobody cares
       cache_->DisownData();
     }
+  }
 
   Slice AllocateKey(std::unique_ptr<const char[]>* key_guard) {
     key_guard->reset(new char[key_size_]);