Update the fl_update_clipboard function, so that it matches the latest

version of the win32 clipboard patch available from
http://www.fltk.org/str.php?L2636. See bug 3386240. 



git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@4625 3789f03b-4d11-0410-bbf8-ca57d06f2519

common/fltk/src/Fl_win32.cxx

@@ -553,11 +553,12 @@ public:
 };
 
 void fl_update_clipboard(void) {
-  HWND hwnd = fl_xid(Fl::first_window());
-
-  if (!hwnd)
+  Fl_Window *w1 = Fl::first_window();
+  if (!w1)
     return;
 
+  HWND hwnd = fl_xid(w1);
+
   if (!OpenClipboard(hwnd))
     return;