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<...>)
#3 librocksdb_src_rocksdb_lib.so+0x1382d88 rocksdb::ForwardIterator::RebuildIterators(bool)
#4 librocksdb_src_rocksdb_lib.so+0x1382362 rocksdb::ForwardIterator::ForwardIterator(rocksdb::DBImpl*, rocksdb::ReadOptions const&, rocksdb::ColumnFamilyData*, rocksdb::SuperVersion*)
#5 librocksdb_src_rocksdb_lib.so+0x11f433f rocksdb::DBImpl::NewIterator(rocksdb::ReadOptions const&, rocksdb::ColumnFamilyHandle*)
#6 rocksdb/src/include/rocksdb/db.h:382 rocksdb::DB::NewIterator(rocksdb::ReadOptions const&)
#7 rocksdb/db_range_del_test.cc:807 rocksdb::DBRangeDelTest_TailingIteratorRangeTombstoneUnsupported_Test::TestBody()
#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 https://github.com/facebook/rocksdb/pull/3300
Differential Revision: D6612989
Pulled By: ajkr
fbshipit-source-id: e7ea2ed914c1b80a8a29d71d92440a6bd9cbcc80
f00e176c5b