windowMenu: Check if monitorIndex is valid before using it

Calling meta_window_get_monitor on an unmanaged window may return
-1, so we need to check the return value.

https://bugzilla.gnome.org/show_bug.cgi?id=788882

js/ui/windowMenu.js

@@ -128,10 +128,9 @@ var WindowMenu = new Lang.Class({
 
         let screen = global.screen;
         let nMonitors = screen.get_n_monitors();
-        if (nMonitors > 1) {
-            this.addMenuItem(new PopupMenu.PopupSeparatorMenuItem());
-
         let monitorIndex = window.get_monitor();
+        if (nMonitors > 1 && monitorIndex >= 0) {
+            this.addMenuItem(new PopupMenu.PopupSeparatorMenuItem());
 
             let dir = Meta.ScreenDirection.UP;
             let upMonitorIndex =