Ignore WM_MENUSELECT messages indicating that the menu was closed.

Don't update the help string in this case, this is not necessary and can clear
the help string set elsewhere.

See #15315.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74467 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775