UserMenu: fix disconnecting a signal that was never connected

If a disabled account is removed, we receive account-removed without
ever seeing account-enabled, and thus _changingId is undefined.

https://bugzilla.gnome.org/show_bug.cgi?id=681382

js/ui/userMenu.js

@@ -654,8 +654,10 @@ const UserMenuButton = new Lang.Class({
     },
 
     _onAccountRemoved: function(accountMgr, account) {
+        if (account._changingId) {
             account.disconnect(account._changingId);
             account._changingId = 0;
+        }
         this._updateChangingPresence();
     },