MergingIterator assertion

Summary: I wrote a test that triggers assertion in MergingIterator. I have not touched that code ever, so I'm looking for somebody with good understanding of the MergingIterator code to fix this. The solution is probably a one-liner. Let me know if you're willing to take a look.

Test Plan: This test fails with an assertion `use_heap_ == false`

Reviewers: dhruba, haobo, sdong, kailiu

Reviewed By: sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16521