Fix recently introduced  assert when using owner drawn items in wxMSW.

Relax the check added in r76202, the item might not be added to its parent
menu yet even if the menu is not NULL.

See #13878.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76251 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775