fix ForwardIterator reference to temporary object

Summary:
Fixes the following ASAN error:

```
==2108042==ERROR: AddressSanitizer: stack-use-after-scope on address 0x7fc50ae9b868 at pc 0x7fc5112aff55 bp 0x7fff9eb9dc10 sp 0x7fff9eb9dc08
=== How to use this, how to get the raw stack trace, and more: fburl.com/ASAN ===
READ of size 8 at 0x7fc50ae9b868 thread T0
SCARINESS: 23 (8-byte-read-stack-use-after-scope)
     #0 rocksdb/dbformat.h:164                   rocksdb::InternalKeyComparator::user_comparator() const
     #1 librocksdb_src_rocksdb_lib.so+0x1429a7d  rocksdb::RangeDelAggregator::InitRep(std::vector<...> const&)
     #2 librocksdb_src_rocksdb_lib.so+0x142ceae  rocksdb::RangeDelAggregator::AddTombstones(std::unique_ptr<...>)
     facebook#3 librocksdb_src_rocksdb_lib.so+0x1382d88  rocksdb::ForwardIterator::RebuildIterators(bool)
     facebook#4 librocksdb_src_rocksdb_lib.so+0x1382362  rocksdb::ForwardIterator::ForwardIterator(rocksdb::DBImpl*, rocksdb::ReadOptions const&, rocksdb::ColumnFamilyData*, rocksdb::SuperVersion*)
     facebook#5 librocksdb_src_rocksdb_lib.so+0x11f433f  rocksdb::DBImpl::NewIterator(rocksdb::ReadOptions const&, rocksdb::ColumnFamilyHandle*)
     facebook#6 rocksdb/src/include/rocksdb/db.h:382     rocksdb::DB::NewIterator(rocksdb::ReadOptions const&)
     facebook#7 rocksdb/db_range_del_test.cc:807         rocksdb::DBRangeDelTest_TailingIteratorRangeTombstoneUnsupported_Test::TestBody()
    facebook#18 rocksdb/db_range_del_test.cc:1006        main

Address 0x7fc50ae9b868 is located in stack of thread T0 at offset 104 in frame
     #0 librocksdb_src_rocksdb_lib.so+0x13825af  rocksdb::ForwardIterator::RebuildIterators(bool)
```
Closes facebook#3300

Differential Revision: D6612989

Pulled By: ajkr

fbshipit-source-id: e7ea2ed914c1b80a8a29d71d92440a6bd9cbcc80

db/forward_iterator.cc

@@ -566,7 +566,7 @@ void ForwardIterator::RebuildIterators(bool refresh_sv) {
     sv_ = cfd_->GetReferencedSuperVersion(&(db_->mutex_));
   }
   RangeDelAggregator range_del_agg(
-      InternalKeyComparator(cfd_->internal_comparator()), {} /* snapshots */);
+      cfd_->internal_comparator(), {} /* snapshots */);
   mutable_iter_ = sv_->mem->NewIterator(read_options_, &arena_);
   sv_->imm->AddIterators(read_options_, &imm_iters_, &arena_);
   if (!read_options_.ignore_range_deletions) {
@@ -621,7 +621,7 @@ void ForwardIterator::RenewIterators() {
   mutable_iter_ = svnew->mem->NewIterator(read_options_, &arena_);
   svnew->imm->AddIterators(read_options_, &imm_iters_, &arena_);
   RangeDelAggregator range_del_agg(
-      InternalKeyComparator(cfd_->internal_comparator()), {} /* snapshots */);
+      cfd_->internal_comparator(), {} /* snapshots */);
   if (!read_options_.ignore_range_deletions) {
     std::unique_ptr<InternalIterator> range_del_iter(
         svnew->mem->NewRangeTombstoneIterator(read_options_));