Fix valgrind complaint about initialization

Summary: Closes https://github.com/facebook/rocksdb/pull/2697

Differential Revision: D5573894

Pulled By: maysamyabandeh

fbshipit-source-id: 8fc03ea8ea6f3f3bc0f68b64cf90243a70562dc4

include/rocksdb/utilities/transaction_db.h

@@ -77,7 +77,7 @@ struct TransactionDBOptions {
   // write only the committed data (WRITE_COMMITTED). The data could be written
   // before the commit phase. The DB then needs to provide the mechanisms to
   // tell apart committed from uncommitted data.
-  TxnDBWritePolicy write_policy;
+  TxnDBWritePolicy write_policy = TxnDBWritePolicy::WRITE_COMMITTED;
 };
 
 struct TransactionOptions {