os: fix regression with handling of nil *File

os: fix regression with handling of nil *File

Use of a nil *File as an argument should not result in a panic,
but result in the ErrInvalid error being returned.
Fix the copy_file_range implementation to preserve this semantic.

Fixes #40115

Change-Id: Iad5ac39664a3efb7964cf55685be636940a8db13
Reviewed-on: https://go-review.googlesource.com/c/go/+/241417
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>