This allows callers to invoke f.Usage() themselves and get the default
usage handler instead of a panic (from calling a nil function).

Fixes #16955.

Change-Id: Ie337fd9e1f85daf78c5eae7b5c41d5ad8c1f89bf
Reviewed-on: https://go-review.googlesource.com/31576
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>