userMenu: Close menu immediately on user/session switch

The same reasoning as for commit 1f30670c1d also applies to
VT switches.

https://bugzilla.gnome.org/show_bug.cgi?id=684459

js/ui/userMenu.js

@@ -820,6 +820,7 @@ const UserMenuButton = new Lang.Class({
     },
 
     _onLoginScreenActivate: function() {
+        this.menu.close(BoxPointer.PopupAnimation.NONE);
         Main.overview.hide();
         if (this._screenSaverSettings.get_boolean(LOCK_ENABLED_KEY))
             Main.screenShield.lock(false);