windows: use cgo.Handle for service object

This fixes go vet complaints and checkptr crashes when the handler
receives an event, e.g. a session change notification.

Fixes golang/go#59687

windows/svc/service.go

@@ -10,6 +10,7 @@ package svc
 
 import (
 	"errors"
+	"runtime/cgo"
 	"sync"
 	"unsafe"
 
@@ -192,7 +193,7 @@ var (
 )
 
 func ctlHandler(ctl, evtype, evdata, context uintptr) uintptr {
-	s := (*service)(unsafe.Pointer(context))
+	s := cgo.Handle(context).Value().(service)
 	e := ctlEvent{cmd: Cmd(ctl), eventType: uint32(evtype), eventData: evdata, context: 123456} // Set context to 123456 to test issue #25660.
 	s.c <- e
 	return 0
@@ -203,7 +204,7 @@ var theService service // This is, unfortunately, a global, which means only one
 // serviceMain is the entry point called by the service manager, registered earlier by
 // the call to StartServiceCtrlDispatcher.
 func serviceMain(argc uint32, argv **uint16) uintptr {
-	handle, err := windows.RegisterServiceCtrlHandlerEx(windows.StringToUTF16Ptr(theService.name), ctlHandlerCallback, uintptr(unsafe.Pointer(&theService)))
+	handle, err := windows.RegisterServiceCtrlHandlerEx(windows.StringToUTF16Ptr(theService.name), ctlHandlerCallback, uintptr(cgo.NewHandle(theService)))
 	if sysErr, ok := err.(windows.Errno); ok {
 		return uintptr(sysErr)
 	} else if err != nil {