fix: do not dereference nil attributes in the example Filecmd

[labeedahmad-debug]

Fixes #661.

Request.Attributes() returns nil when the attribute blob does not hold every field its flags promise. That is deliberate — #325 / #328 replaced a panic inside the parser with a nil return — but request-example.go dereferences it:

if r.AttrFlags().Size {
	return file.Truncate(int64(r.Attributes().Size))
}

A SSH_FXP_SETSTAT that sets SSH_FILEXFER_ATTR_SIZE and carries fewer than eight bytes of attributes reaches that line with nil. The panic happens in a packetWorker goroutine, so the caller cannot recover it and the process ends. I found it by fuzzing a server whose Filecmd started from this example.

What this changes

• request-example.go: answer ErrSSHFxBadMessage when Attributes() is nil, instead of dereferencing it.
• request-attrs.go: say in the doc comment that Attributes() returns nil for malformed attributes, so callers know they have to check. The comment gave no hint, and the example taught the unchecked pattern.
• request-example_test.go (new): a regression test that drives the example handler with a short attribute blob. It panics without the fix and passes with it. It calls the handler directly rather than through clientRequestServerPair, so it also runs on Windows and Plan 9.

The library's own parsing is unchanged; only the example and a doc comment.

Verification

go test -run TestInMemHandlerSetstatShortAttributes -v .
=== RUN   TestInMemHandlerSetstatShortAttributes
--- PASS: TestInMemHandlerSetstatShortAttributes (0.00s)

Reverting only the request-example.go hunk makes that test panic at request-example.go:160, which is the reported crash.

go test ./... is otherwise unchanged by this PR. Note that TestCleanPath already fails on master on windows/amd64, before and after this change, so it looks unrelated and platform-specific.

🤖 Generated with Claude Code

[puellanivis]

Please do not use Co-Authored-By for an LLM. Legally, an LLM cannot be an author in any copyright jurisdiction that I am aware of. Instead, it is suggested to use Assisted-by:: https://docs.kernel.org/process/coding-assistants.html#attribution

[puellanivis]

There is no reason to type assert out to the concrete *root type here. We can just use the FilePut and ̀ FileCmd` fields as they’re designed to be used:

Suggested change

	fs := InMemHandler().FileCmd.(*root)
	_, err := fs.Filewrite(&Request{Method: "Put", Filepath: "/foo", Flags: sshFxfWrite | sshFxfCreat})
	require.NoError(t, err)
	err = fs.Filecmd(&Request{
	fs := InMemHandler()
	_, err := fs.FilePut.Filewrite(&Request{Method: "Put", Filepath: "/foo", Flags: sshFxfWrite | sshFxfCreat})
	require.NoError(t, err)
	err = fs.FileCmd.Filecmd(&Request{

[puellanivis]

I’m going to recommend we provide a better example here, because if one were supporting more than one setstat operation, it would need to be repeated. Instead, let’s test the attributes up front:

		attrs := r.Attributes()
		if attrs == nil {
			// Something went wrong parsing attributes
			return ErrSSHFxBadMessage
		}

		if r.AttrFlags().Size {
			return file.Truncate(int64(attrs.Size))
		}