Fix crash when destroying wxPopupWindow without hiding it first

The global wxCurrentPopupWindow used by wxMSW wasn't reset in this case
and resulted in a crash during the next mouse or focus event.

Do it in wxPopupWindow dtor now, and not just when the popup is hidden.

include/wx/msw/popupwin.h

@@ -25,6 +25,8 @@ public:
 
     bool Create(wxWindow *parent, int flags = wxBORDER_NONE);
 
+    virtual ~wxPopupWindow();
+
     virtual void SetFocus() wxOVERRIDE;
     virtual bool Show(bool show = true) wxOVERRIDE;
 

src/msw/popupwin.cpp

@@ -59,6 +59,14 @@ bool wxPopupWindow::Create(wxWindow *parent, int flags)
                                 flags);
 }
 
+wxPopupWindow::~wxPopupWindow()
+{
+    // If the popup is destroyed without being hidden first, ensure that we are
+    // not left with a dangling pointer.
+    if ( wxCurrentPopupWindow == this )
+        wxCurrentPopupWindow = NULL;
+}
+
 WXDWORD wxPopupWindow::MSWGetStyle(long flags, WXDWORD *exstyle) const
 {
     // we only honour the border flags, the others don't make sense for us