Fix WritePreparedTransactionTest::SeqAdvanceTest ASAN failure

Summary: Closes https://github.com/facebook/rocksdb/pull/2922

Differential Revision: D5895310

Pulled By: yiwu-arbug

fbshipit-source-id: 52c635a25d22478ec1eca49b6817551202babac2

utilities/transactions/write_prepared_transaction_test.cc

@@ -621,6 +621,7 @@ TEST_P(WritePreparedTransactionTest, SeqAdvanceTest) {
   ASSERT_OK(s);
   seq = db_impl->GetLatestSequenceNumber();
   ASSERT_EQ(exp_seq, seq);
+  delete txn;
 
   // Commit without prepare. It shoudl write to DB without a commit marker.
   txn = db->BeginTransaction(write_options, txn_options);