Fix Get does not return super version on error

Summary:
This is caught when I was testing #2886.
Closes https://github.com/facebook/rocksdb/pull/2907

Differential Revision: D5863153

Pulled By: yiwu-arbug

fbshipit-source-id: 8c54759ba1a0dc101f24ab50423e35731300612d

db/db_impl.cc

@@ -969,6 +969,7 @@ Status DBImpl::GetImpl(const ReadOptions& read_options,
       RecordTick(stats_, MEMTABLE_HIT);
     }
     if (!done && !s.ok() && !s.IsMergeInProgress()) {
+      ReturnAndCleanupSuperVersion(cfd, sv);
       return s;
     }
   }