Remove assert(current_ == CurrentReverse()) in MergingIterator::Prev()

Summary:
Remove assert(current_ == CurrentReverse()) in MergingIterator::Prev()
because it is possible to have some keys larger than the seek-key
inserted between Seek() and SeekToLast(), which makes current_ not
equal to CurrentReverse().

Test Plan: db_stress

Reviewers: igor, sdong, IslamAbdelRahman, anthony

Reviewed By: anthony

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D41331

table/merger.cc

@@ -186,9 +186,12 @@ class MergingIterator : public Iterator {
         }
       }
       direction_ = kReverse;
-      // The loop retreated all non-current children to be < key() so current_
+      // Note that we don't do assert(current_ == CurrentReverse()) here
-      // should still be strictly the largest key.
+      // because it is possible to have some keys larger than the seek-key
-      assert(current_ == CurrentReverse());
+      // inserted between Seek() and SeekToLast(), which makes current_ not
+      // equal to CurrentReverse().
+      //
+      // assert(current_ == CurrentReverse());
     }
 
     current_->Prev();