Check window validity in wxNonOwnedWindow::Update() in wxMac too

This was already done in the other functions, but not this one,
resulting in a crash if it was called for an object with null m_nowpeer
(e.g. before it's created or when it's not a real wxNonOwnedWindow
object as when it's a subobject of wxTDIChildFrame).

See #18423.

src/osx/nonownedwnd_osx.cpp

@@ -481,6 +481,9 @@ void wxNonOwnedWindow::WindowWasPainted()
 
 void wxNonOwnedWindow::Update()
 {
+    if ( m_nowpeer == NULL )
+        return;
+
     if ( clock() - s_lastFlush > CLOCKS_PER_SEC / 30 )
     {
         s_lastFlush = clock();