setup wal_in_db_path_ for secondary instance (#5545)

Summary:
PR https://github.com/facebook/rocksdb/pull/5520 adds DBImpl:: wal_in_db_path_ and initializes it in DBImpl::Open, this PR fixes the valgrind error for secondary instance:
```
==236417== Conditional jump or move depends on uninitialised value(s)
==236417==    at 0x62242A: rocksdb::DeleteDBFile(rocksdb::ImmutableDBOptions const*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, bool, bool) (file_util.cc:96)
==236417==    by 0x512432: rocksdb::DBImpl::DeleteObsoleteFileImpl(int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, rocksdb::FileType, unsigned long) (db_impl_files.cc:261)
==236417==    by 0x515A7A: rocksdb::DBImpl::PurgeObsoleteFiles(rocksdb::JobContext&, bool) (db_impl_files.cc:492)
==236417==    by 0x499153: rocksdb::ColumnFamilyHandleImpl::~ColumnFamilyHandleImpl() (column_family.cc:75)
==236417==    by 0x499880: rocksdb::ColumnFamilyHandleImpl::~ColumnFamilyHandleImpl() (column_family.cc:84)
==236417==    by 0x4C9AF9: rocksdb::DB::DestroyColumnFamilyHandle(rocksdb::ColumnFamilyHandle*) (db_impl.cc:3105)
==236417==    by 0x44E853: CloseSecondary (db_secondary_test.cc:53)
==236417==    by 0x44E853: rocksdb::DBSecondaryTest::~DBSecondaryTest() (db_secondary_test.cc:31)
==236417==    by 0x44EC77: ~DBSecondaryTest_PrimaryDropColumnFamily_Test (db_secondary_test.cc:443)
==236417==    by 0x44EC77: rocksdb::DBSecondaryTest_PrimaryDropColumnFamily_Test::~DBSecondaryTest_PrimaryDropColumnFamily_Test() (db_secondary_test.cc:443)
==236417==    by 0x83D1D7: HandleSehExceptionsInMethodIfSupported<testing::Test, void> (gtest-all.cc:3824)
==236417==    by 0x83D1D7: void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) (gtest-all.cc:3860)
==236417==    by 0x8346DB: testing::TestInfo::Run() [clone .part.486] (gtest-all.cc:4078)
==236417==    by 0x8348D4: Run (gtest-all.cc:4047)
==236417==    by 0x8348D4: testing::TestCase::Run() [clone .part.487] (gtest-all.cc:4190)
==236417==    by 0x834D14: Run (gtest-all.cc:6100)
==236417==    by 0x834D14: testing::internal::UnitTestImpl::RunAllTests() (gtest-all.cc:6062)
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5545

Differential Revision: D16146224

Pulled By: miasantreble

fbshipit-source-id: 184c90e451352951da4e955f054d4b1a1f29ea29

db/db_impl/db_impl_secondary.cc

@@ -587,6 +587,9 @@ Status DB::OpenAsSecondary(
       &impl->write_controller_));
   impl->column_family_memtables_.reset(
       new ColumnFamilyMemTablesImpl(impl->versions_->GetColumnFamilySet()));
+  impl->wal_in_db_path_ =
+      IsWalDirSameAsDBPath(&impl->immutable_db_options_);
+
   impl->mutex_.Lock();
   s = impl->Recover(column_families, true, false, false);
   if (s.ok()) {