BUG: Fix regression in groupby.rolling.corr/cov when other is same size as each group
- [x] closes #42915
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

nice ping on greenish greenish thanks @mroeschke  @meeseeksdev backport 1.3.x Something went wrong ... Please have a look at my logs.