Correct use of GetCurrentMessageLoopThreadId is racy

[cfergeau]

GetCurrentMessageLoopThreadId documentation says:

//  Returns [...] "0" if [a message loop] is not running. The latter indicates a mistake, 
// as this function should only be called after a call to one of the _OnQuit functions.

The _OnQuit functions create the message loop in a go routine (see

winquit/pkg/winquit/server_windows.go

Lines 64 to 68 in d75df78

	func initLoop() {
	loopInit.Do(func() {
	go messageLoop()
	})
	}

), so when these functions return, it's not 100% guaranteed that a message loop is running and that it has a thread id.

We hit that race in gvisor-tap-vsock with test failures when running in a gh actions runner: containers/gvisor-tap-vsock#433