In the case of a file being deleted while Readdir was running, it was
possible for File.Readdir to return an empty slice and a nil error,
counter to its documentation.

Fixes #16919

Change-Id: If0e42882eea52fbf5530317a1895f3829ea8e67b
Reviewed-on: https://go-review.googlesource.com/28056Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>