Fix memorty leak in `rocksdb_wal_iter_get_batch` function (#5515)

Summary:
`wal_batch.writeBatchPtr.release()` gives up the ownership of the original `WriteBatch`, but there is no new owner, which causes memory leak.

The patch is simple. Removing `release()` prevent ownership change. `std::move` is for speed.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5515

Differential Revision: D16264281

Pulled By: riversand963

fbshipit-source-id: 51c556b7a1c977325c3aa24acb636303847151fa

db/c.cc

@@ -1034,7 +1034,7 @@ void rocksdb_wal_iter_destroy (const rocksdb_wal_iterator_t* iter) {
 rocksdb_writebatch_t* rocksdb_wal_iter_get_batch (const rocksdb_wal_iterator_t* iter, uint64_t* seq) {
   rocksdb_writebatch_t* result = rocksdb_writebatch_create();
   BatchResult wal_batch = iter->rep->GetBatch();
-  result->rep = * wal_batch.writeBatchPtr.release();
+  result->rep = std::move(*wal_batch.writeBatchPtr);
   if (seq != nullptr) {
     *seq = wal_batch.sequence;
   }