os: regression with nil handling and copy_file_range

[dsnet]

Consider the following snippet:

var src *os.File
dst, _ := os.Create("/tmp/file")
fmt.Println(io.Copy(dst, src))

On go1.14, this prints:

0 invalid argument

On go1.15, this panics with:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x490cc1]

goroutine 1 [running]:
os.(*File).readFrom(0xc000102018, 0x4dddc0, 0x0, 0x54da28, 0x1000000000000, 0x7f1f86dbcfe8, 0xc0000a0e70)
	os/readfrom_linux.go:36 +0x81
os.(*File).ReadFrom(0xc000102018, 0x4dddc0, 0x0, 0x7f1f86dbcfe8, 0xc000102018, 0x490301)
	os/file.go:151 +0x5c
io.copyBuffer(0x4ddde0, 0xc000102018, 0x4dddc0, 0x0, 0x0, 0x0, 0x0, 0xc000034778, 0xc0000a0f78, 0x405585)
	io/io.go:395 +0x2ff
io.Copy(...)
	io/io.go:368
main.main()
	main.go:15 +0x8c

[dsnet]

\cc @ianlancetaylor @neild

[gopherbot]

Change https://golang.org/cl/241417 mentions this issue: os: fix regression with handling of nil *File