Fix TransactionTest::SeqAdvanceTest ASAN failure
Fix TransactionTest::SeqAdvanceTest ASAN failure

Summary:
The test didn't delete txn before creating a new one.
Closes https://github.com/facebook/rocksdb/pull/2913

Differential Revision: D5880236

Pulled By: yiwu-arbug

fbshipit-source-id: 7a4fcaada3d86332292754502cd8f4341143bf4f

SunguckLee/RocksDB,SunguckLee/RocksDB,Andymic/rocksdb,Andymic/rocksdb,Andymic/rocksdb,bbiao/rocksdb,Andymic/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,Andymic/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,Andymic/rocksdb,bbiao/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,bbiao/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb