Fix Clang release build broken by 5582123d

Summary:
5582123d broken CLANG release build because of an unexpected change. Fix it.
Closes https://github.com/facebook/rocksdb/pull/2443

Differential Revision: D5236297

Pulled By: siying

fbshipit-source-id: 1b410adf13ded149c53e8235e9ea9f3130fb5403

db/version_set.cc

@@ -228,7 +228,9 @@ class FilePicker {
   unsigned int hit_file_level_;
   int32_t search_left_bound_;
   int32_t search_right_bound_;
+#ifndef NDEBUG
   std::vector<FileMetaData*>* files_;
+#endif
   autovector<LevelFilesBrief>* level_files_brief_;
   bool search_ended_;
   bool is_hit_file_last_in_level_;