fix populating range deletions in forward iterator

Summary:
fixes #2902
Closes https://github.com/facebook/rocksdb/pull/2917

Differential Revision: D5887175

Pulled By: ajkr

fbshipit-source-id: 364e292c636a3238bfc53b0fb9a01ff2f82dcbb9

db/forward_iterator.cc

@@ -626,7 +626,7 @@ void ForwardIterator::RenewIterators() {
     std::unique_ptr<InternalIterator> range_del_iter(
         svnew->mem->NewRangeTombstoneIterator(read_options_));
     range_del_agg.AddTombstones(std::move(range_del_iter));
-    sv_->imm->AddRangeTombstoneIterators(read_options_, &arena_,
+    svnew->imm->AddRangeTombstoneIterators(read_options_, &arena_,
                                            &range_del_agg);
   }