Small fix in WBWIMemtable::UpdateKey() (facebook#13171)

Summary:
fixes issue facebook#13166.

Pull Request resolved: facebook#13171

Test Plan: the same command in facebook#13166 doesn't reproduce for me.

Reviewed By: ltamasi

Differential Revision: D66621871

Pulled By: cbi42

fbshipit-source-id: 08820a22071091606b437181e2b5e9343202d637

memtable/wbwi_memtable.h

@@ -123,7 +123,7 @@ class WBWIMemTableIterator final : public InternalIterator {
       key_.clear();
       valid_ = false;
       s_ = Status::Corruption("Unexpected write_batch_with_index entry type " +
-                              std::to_string(t->second));
+                              std::to_string(it_->Entry().type));
       return;
     }
     key_buf_.SetInternalKey(it_->Entry().key, global_seqno_, t->second);