If a filepath.WalkFunc is called with an non-nil err argument, it's possible
that the info argument will be nil. The comment above filepath.WalkFunc now
reflects this.

Fixes #26425

Change-Id: Ib9963b3344587d2993f1698c5a801f2d1286856b
GitHub-Last-Rev: 553fc266b570d0c47efe12b3b670f88112e3b334
GitHub-Pull-Request: golang/go#26435
Reviewed-on: https://go-review.googlesource.com/124635Reviewed-by: Ian Lance Taylor <iant@golang.org>