Summary: Replace exception by setting valid_ = false in DBIter::MergeValuesNewToOld().

Test Plan:
Not sure if I am right at this, but it seems we currently don't have a good
way to test that code path as it requires dynamically set merge_operator = nullptr
at the time while Merge() is calling.

Reviewers: igor, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D29811