BUG: DataFrame.loc returning empty result with negative stepsize for MultiIndex
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

This was easier than thought. Should I add a whatsnew note for this?
yes pls add a whatsnew note (1.2. bugs for MI / indexing), merge master and ping on green. @jreback green