BUG: IntervalIndex.get_indexer with categorical both have nans
- [ ] closes #xxxx
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

Started with moving a fastpath from IntervalIndex to the base class.  In the process found a bug in the IntervalIndex version, which is now fixed.