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

db/db_test.cc

@@ -1334,6 +1334,23 @@ TEST(DBTest, FilterDeletes) {
   } while (ChangeCompactOptions());
 }
 
+
+TEST(DBTest, IterSeekBeforePrev) {
+  ASSERT_OK(Put("a", "b"));
+  ASSERT_OK(Put("c", "d"));
+  dbfull()->Flush(FlushOptions());
+  ASSERT_OK(Put("0", "f"));
+  ASSERT_OK(Put("1", "h"));
+  dbfull()->Flush(FlushOptions());
+  ASSERT_OK(Put("2", "j"));
+  auto iter = db_->NewIterator(ReadOptions());
+  iter->Seek(Slice("c"));
+  iter->Prev();
+  iter->Seek(Slice("a"));
+  iter->Prev();
+  delete iter;
+}
+
 TEST(DBTest, IterEmpty) {
   do {
     Iterator* iter = db_->NewIterator(ReadOptions());