Correct out-of-bounds error with large indeces
- [x] closes #12527 
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

the build is pending for 13 hours now ...
 yeah I think that the OSX queue was bogged for a while; these _seem_ to be proceeding now (though still quite a backlog)
 @jreback the build is green
 thanks!
