Summary:
After deleting file iterators which are over the iterate upper
bound, we also need to check for null pointers in
ResetIncompletIterators.

Test Plan: db_tailing_iter_test.TailingIteratorTrimSeekToNext

Reviewers: tnovak, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D45525