gdkevents-win32.c: Avoid crash in SetTimer()

Since we only support a single GdkDisplay for Windows, make use of it to
ensure we always obtain a proper GdkDisplay to check whether a modal
operation is in progress.

This should fix issue #7147.

gdk/win32/gdkevents-win32.c

@@ -1365,7 +1365,9 @@ modal_timer_proc (HWND     hwnd,
 		  DWORD    time)
 {
   int arbitrary_limit = 10;
-  GdkWin32Display *display = GDK_WIN32_DISPLAY (gdk_surface_get_display (GDK_SURFACE (id)));
+
+  /* todo: if we support multiple GdkDisplay's on Windows? */
+  GdkWin32Display *display = GDK_WIN32_DISPLAY (gdk_display_get_default ());
 
   while (display->display_surface_record->modal_operation_in_progress != GDK_WIN32_MODAL_OP_NONE &&
 	 g_main_context_pending (NULL) &&