Fixed #10414 -- Make select_related fail on invalid field names.

https://code.djangoproject.com/ticket/10414

This is basically my first non-trivial contribution to Django, so it might not be perfect. I'll be happy to update the PR as required. :)
Basically a PEP8 E128 fix. However, that particular rule is ignored by the CI, so it is not strictly a necessary fix.