Change m_windowCurrent to m_currentNSWindow such that m_currentNSWindow is

Change m_windowCurrent to m_currentNSWindow such that m_currentNSWindow is
never used directly but only to look up the associated wxCocoaNSWindow.
If the wxCocoaNSWindow cannot be found later in UpdateMenuBar then it's
likely been destroyed but no crash will result as before.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@26720 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775