Initialize `lowest_unnecessary_level_` in `VersionStorageInfo` constructor (#11359)

Summary:
valgrind complains "Conditional jump or move depends on uninitialised value(s)". A sample error message:

```
[ RUN      ] DBCompactionTest.DrainUnnecessaryLevelsAfterDBBecomesSmall
==3353864== Conditional jump or move depends on uninitialised value(s)
==3353864==    at 0x8647B4: rocksdb::VersionStorageInfo::ComputeCompactionScore(rocksdb::ImmutableOptions const&, rocksdb::MutableCFOptions const&) (version_set.cc:3414)
==3353864==    by 0x86B340: rocksdb::VersionSet::AppendVersion(rocksdb::ColumnFamilyData*, rocksdb::Version*) (version_set.cc:4946)
==3353864==    by 0x876B88: rocksdb::VersionSet::CreateColumnFamily(rocksdb::ColumnFamilyOptions const&, rocksdb::VersionEdit const*) (version_set.cc:6876)
==3353864==    by 0xBA66FE: rocksdb::VersionEditHandler::CreateCfAndInit(rocksdb::ColumnFamilyOptions const&, rocksdb::VersionEdit const&) (version_edit_handler.cc:483)
==3353864==    by 0xBA4A81: rocksdb::VersionEditHandler::Initialize() (version_edit_handler.cc:187)
==3353864==    by 0xBA3927: rocksdb::VersionEditHandlerBase::Iterate(rocksdb::log::Reader&, rocksdb::Status*) (version_edit_handler.cc:31)
==3353864==    by 0x870173: rocksdb::VersionSet::Recover(std::vector<rocksdb::ColumnFamilyDescriptor, std::allocator<rocksdb::ColumnFamilyDescriptor> > const&, bool, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*, bool) (version_set.cc:5729)
==3353864==    by 0x7538FA: rocksdb::DBImpl::Recover(std::vector<rocksdb::ColumnFamilyDescriptor, std::allocator<rocksdb::ColumnFamilyDescriptor> > const&, bool, bool, bool, unsigned long*, rocksdb::DBImpl::RecoveryContext*) (db_impl_open.cc:522)
==3353864==    by 0x75BA0F: rocksdb::DBImpl::Open(rocksdb::DBOptions const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::vector<rocksdb::ColumnFamilyDescriptor, std::allocator<rocksdb::ColumnFamilyDescriptor> > const&, std::vector<rocksdb::ColumnFamilyHandle*, std::allocator<rocksdb::ColumnFamilyHandle*> >*, rocksdb::DB**, bool, bool) (db_impl_open.cc:1928)
==3353864==    by 0x75A735: rocksdb::DB::Open(rocksdb::DBOptions const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::vector<rocksdb::ColumnFamilyDescriptor, std::allocator<rocksdb::ColumnFamilyDescriptor> > const&, std::vector<rocksdb::ColumnFamilyHandle*, std::allocator<rocksdb::ColumnFamilyHandle*> >*, rocksdb::DB**) (db_impl_open.cc:1743)
==3353864==    by 0x75A510: rocksdb::DB::Open(rocksdb::Options const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, rocksdb::DB**) (db_impl_open.cc:1720)
==3353864==    by 0x5925FD: rocksdb::DBTestBase::TryReopen(rocksdb::Options const&) (db_test_util.cc:710)
==3353864==  Uninitialised value was created by a heap allocation
==3353864==    at 0x4842F0F: operator new(unsigned long) (vg_replace_malloc.c:422)
==3353864==    by 0x876AF4: rocksdb::VersionSet::CreateColumnFamily(rocksdb::ColumnFamilyOptions const&, rocksdb::VersionEdit const*) (version_set.cc:6870)
==3353864==    by 0xBA66FE: rocksdb::VersionEditHandler::CreateCfAndInit(rocksdb::ColumnFamilyOptions const&, rocksdb::VersionEdit const&) (version_edit_handler.cc:483)
==3353864==    by 0xBA4A81: rocksdb::VersionEditHandler::Initialize() (version_edit_handler.cc:187)
==3353864==    by 0xBA3927: rocksdb::VersionEditHandlerBase::Iterate(rocksdb::log::Reader&, rocksdb::Status*) (version_edit_handler.cc:31)
==3353864==    by 0x870173: rocksdb::VersionSet::Recover(std::vector<rocksdb::ColumnFamilyDescriptor, std::allocator<rocksdb::ColumnFamilyDescriptor> > const&, bool, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*, bool) (version_set.cc:5729)
==3353864==    by 0x7538FA: rocksdb::DBImpl::Recover(std::vector<rocksdb::ColumnFamilyDescriptor, std::allocator<rocksdb::ColumnFamilyDescriptor> > const&, bool, bool, bool, unsigned long*, rocksdb::DBImpl::RecoveryContext*) (db_impl_open.cc:522)
==3353864==    by 0x75BA0F: rocksdb::DBImpl::Open(rocksdb::DBOptions const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::vector<rocksdb::ColumnFamilyDescriptor, std::allocator<rocksdb::ColumnFamilyDescriptor> > const&, std::vector<rocksdb::ColumnFamilyHandle*, std::allocator<rocksdb::ColumnFamilyHandle*> >*, rocksdb::DB**, bool, bool) (db_impl_open.cc:1928)
==3353864==    by 0x75A735: rocksdb::DB::Open(rocksdb::DBOptions const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::vector<rocksdb::ColumnFamilyDescriptor, std::allocator<rocksdb::ColumnFamilyDescriptor> > const&, std::vector<rocksdb::ColumnFamilyHandle*, std::allocator<rocksdb::ColumnFamilyHandle*> >*, rocksdb::DB**) (db_impl_open.cc:1743)
==3353864==    by 0x75A510: rocksdb::DB::Open(rocksdb::Options const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, rocksdb::DB**) (db_impl_open.cc:1720)
==3353864==    by 0x5925FD: rocksdb::DBTestBase::TryReopen(rocksdb::Options const&) (db_test_util.cc:710)
==3353864==    by 0x591F73: rocksdb::DBTestBase::Reopen(rocksdb::Options const&) (db_test_util.cc:662)
```

This is likely about `lowest_unnecessary_level_` even though it would be initialized in `CalculateBaseBytes()` before being used in `ComputeCompactionScore()`. Initialize it also in VersionStorageInfo constructor to prevent valgrind from  complaining.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11359

Test Plan: - ran a test with valgrind which gave the error message above before this PR: `valgrind --track-origins=yes ./db_compaction_test  --gtest_filter="*DrainUnnecessaryLevelsAfterDBBecomesSmall*"`

Reviewed By: hx235

Differential Revision: D44799112

Pulled By: cbi42

fbshipit-source-id: 557208a66f04a2163b418b2a651bdb7e777c4511

db/version_set.cc

@@ -2050,6 +2050,7 @@ VersionStorageInfo::VersionStorageInfo(
       compaction_style_(compaction_style),
       files_(new std::vector<FileMetaData*>[num_levels_]),
       base_level_(num_levels_ == 1 ? -1 : 1),
+      lowest_unnecessary_level_(-1),
       level_multiplier_(0.0),
       files_by_compaction_pri_(num_levels_),
       level0_non_overlapping_(false),