REGR: read_fwf interpreting infer as list of colspecs when checking names length
- [x] closes #45337
- [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
- [ ] whatsnew entry

No whatsnew needed if this goes into 1.4, otherwise I will add one for 1.4.1 when available
thanks @phofl  @meeseeksdev backport 1.4.x Something went wrong ... Please have a look at my logs.