BUG: fix categorical searchsorted bug (#14522)
 - [ ] closes #14522 
 - [ ] tests added / passed
 - [ ] passes ``git diff upstream/master | flake8 --diff``
 - [ ] whatsnew entry

the bug is cause by `x.categories.values.searchsorted`, where the search should not be performed.
@chiangqiqi Can you add tests for this?
 I think I miss understood the meaning, will try to fix this soon.
