Summary:
I was looking at https://github.com/facebook/rocksdb/issues/2636 and got very confused that `MergingIterator::AddIterator()` is populating `min_heap_` with dangling pointers. There is justification in the comments that `min_heap_` will be cleared before it's used, but it'd be cleaner to not populate it with dangling pointers in the first place. Also made similar change in the constructor for consistency, although the pointers there would not be dangling, just unused.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/8975
Test Plan: rely on existing tests
Reviewed By: pdillinger, hx235
Differential Revision: D31273767
Pulled By: ajkr
fbshipit-source-id: 127ca9dd1f82f77f55dd0c3f19511de3282fc229
c0ec58ecb9