Fix out-of-bounds access in MultiDBParallelOpenTest (#9046)

Summary:
`dbs` should not be cleared, as it is reused later when reopening the DBs, so we have an out-of-bounds access with `dbnames[dbnum]`. The values left in the vector don't need to be reset, as the db pointer is an out parameter for `DB::Open`.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9046

Reviewed By: pdillinger

Differential Revision: D31738263

Pulled By: ot

fbshipit-source-id: c619e947b8d3dbc3d896f29971f093d3e3c794d3

db/db_test2.cc

@@ -5537,8 +5537,6 @@ TEST_F(DBTest2, TestCompactFiles) {
 }
 #endif  // ROCKSDB_LITE
 
-// TODO: figure out why this test fails in appveyor
-#ifndef OS_WIN
 TEST_F(DBTest2, MultiDBParallelOpenTest) {
   const int kNumDbs = 2;
   Options options = CurrentOptions();
@@ -5569,7 +5567,6 @@ TEST_F(DBTest2, MultiDBParallelOpenTest) {
   }
 
   // Verify non-empty DBs can be recovered in parallel
-  dbs.clear();
   open_threads.clear();
   for (int i = 0; i < kNumDbs; ++i) {
     open_threads.emplace_back(
@@ -5586,7 +5583,6 @@ TEST_F(DBTest2, MultiDBParallelOpenTest) {
     ASSERT_OK(DestroyDB(dbnames[i], options));
   }
 }
-#endif  // OS_WIN
 
 namespace {
 class DummyOldStats : public Statistics {