fix(implant): services list panics on nil error on Windows (#1989)

[Moriz82]

Bug

Running services against a Windows beacon can kill the beacon with a nil pointer deref. The implant panics and the session dies.

Cause

servicesListHandler in implant/sliver/handlers/handlers_windows.go calls err.Error() with no nil check:

serviceInfo, err := service.ListServices(servicesReq.Hostname)
servicesResp := &sliverpb.Services{
    Details:  serviceInfo,
    Error:    err.Error(),   // nil deref when err == nil
    Response: &commonpb.Response{},
}

ListServices returns (list, nil) when every service reads without error. Calling .Error() on nil → 0xc0000005, process exits.

Fix

Add the nil guard. One helper, one check:

func buildServicesResp(details []*sliverpb.ServiceDetails, err error) *sliverpb.Services {
    errStr := ""
    if err != nil {
        errStr = err.Error()
    }
    return &sliverpb.Services{
        Details:  details,
        Error:    errStr,
        Response: &commonpb.Response{},
    }
}

Why it doesn't repro easily

A beacon running as a normal user hits access-denied on a bunch of services during openService / Config() / Query(). Those failures get collected into serviceErrors and ListServices returns a non-nil error — so the buggy err.Error() call is safe. That's why most operators never see this crash.

The bug only fires when every service reads cleanly (no errors at all). On modern Win10/11 that's hard to hit even as SYSTEM because services like WaaSMedicSvc are PPL-protected and will fail for anyone. But it's not impossible — older Windows versions, stripped-down VMs, or just the right host config hits it and the beacon dies.

Reproducing it deliberately

To force the code path, add one line to ListServices (implant/sliver/service/service_windows.go, near line 195):

operationError = nil // REPRO: force nil error to exercise the bug path

Then build sliver-server, generate a Windows beacon, run it on any Win VM, and call services — the unpatched beacon panics, the patched beacon returns results normally.

Fixes #1989