Improve ClientDriveQueryDirectoryResponse.encode()

[ibeckermayer]

Takes some of the ad hoc "correctness" checking that was built into ClientDriveQueryDirectoryResponse.encode() and moves it to ClientDriveQueryDirectoryResponse.new(), allowing encode() to be significantly simplified.

[zmb3]

Suggested change

	NTSTATUS::STATUS_SUCCESS => {
	if buffer.is_none() {
	NTSTATUS::STATUS_SUCCESS if buffer.is_none() => {

Up to you whether using match guards makes this read cleaner.

[ibeckermayer]

I just tried that, but it turns out that the issue with using a match guard here is that if the match guard fails, then the control gets bumped down to the wildcard match (which isn't what we want in the case of i.e. NTSTATUS::STATUS_SUCCESS && buffer.is_some()):

            _ => {
                return Err(invalid_data_error(&format!(
                    "received unsupported io_status for ClientDriveQueryDirectoryResponse: {:?}",
                    io_status
                )))
            }

So if I were to use a match guard here, I would need to then do something like another match in that wildcard to confirm a legitimate NTSTATUS was received, or add another match case for NTSTATUS::STATUS_SUCCESS if buffer.is_some() (it would read worse).

In a loose sense, adding the match guard sidesteps typical match semantics of matching on all possible values of NTSTATUS, and instead matches on all NTSTATUS && <match guards>.