Don't check the same crossing mode twice

This was broken since commit b2aaa94 in 2008. Its commit message
clearly states that the intention was to check for GTK_GRAB,
GTK_UNGRAB and STATE_CHANGED. Lets do that, then.

This was found by Coverity.

gtk/gtkmenushell.c

@@ -1107,7 +1107,7 @@ gtk_menu_shell_leave_notify (GtkWidget        *widget,
                              GdkEventCrossing *event)
 {
   if (event->mode == GDK_CROSSING_GTK_GRAB ||
-      event->mode == GDK_CROSSING_GTK_GRAB ||
+      event->mode == GDK_CROSSING_GTK_UNGRAB ||
       event->mode == GDK_CROSSING_STATE_CHANGED)
     return TRUE;