os: fix another case where RemoveAll should return nil
os: fix another case where RemoveAll should return nil

This hopefully fixes issue 8793.

Fixes Issue 8793

LGTM=iant
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/150860046

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go