Initialize StressTest::optimistic_txn_db_ in ctor (#11547)

Summary:
`StressTest::optimistic_txn_db_` is currently not initialized by the constructor, which
can lead to assertion failures down the line in `StressTest::Open`.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11547

Reviewed By: cbi42

Differential Revision: D46845658

Pulled By: ltamasi

fbshipit-source-id: 578b0f24fc00e3e97f24221fcdd003cc529439c2

db_stress_tool/db_stress_test_base.cc

@@ -56,7 +56,7 @@ StressTest::StressTest()
       filter_policy_(CreateFilterPolicy()),
       db_(nullptr),
       txn_db_(nullptr),
-
+      optimistic_txn_db_(nullptr),
       db_aptr_(nullptr),
       clock_(db_stress_env->GetSystemClock().get()),
       new_column_family_name_(1),