Don't crash in wxOSXGetImageFromBundle() if the bundle is invalid

Just return a null image instead, as this function can be called when we
don't have any bitmaps too -- as it happens in the Button::Bitmap unit
test, for example.

src/osx/core/bmpbndl.mm

@@ -171,6 +171,9 @@ wxBitmapBundle wxBitmapBundle::FromResources(const wxString& name)
 
 WXImage wxOSXGetImageFromBundle(const wxBitmapBundle& bundle)
 {
+    if (!bundle.IsOk())
+        return NULL;
+
     WXImage image = bundle.GetImpl()->OSXGetImage();
 
     if (image == 0)