BUG: sorting with large float and multiple columns incorrect
Fixes https://github.com/pandas-dev/pandas/issues/14922
    
Having the `int` equivalent of `NaT` in an `int64` column caused wrong sorting because this special value was considered as "missing value".

 - [ ] closes #xxxx
 - [X] tests added / passed
 - [X] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry
