[gnome-shell] ScreenShield: fix code to detect missing unlock dialog

[Giovanni Campagna <gcampagna src gnome org>]

commit 271508c0a871ae508b94e06a7d11985b5998bc12
Author: Giovanni Campagna <gcampagna src gnome org>
Date:   Tue Nov 6 23:41:51 2012 +0100

    ScreenShield: fix code to detect missing unlock dialog
    
    If the session mode has no locking support, screenshield had code to
    unlock automatically, but it did so by checking the return value of
    the constructor, instead of checking if the constructor was actually
    callable, so it would get a TypeError before reaching the check.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=687708

 js/ui/screenShield.js |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/js/ui/screenShield.js b/js/ui/screenShield.js
index 7c7c7c0..8de5c62 100644
--- a/js/ui/screenShield.js
+++ b/js/ui/screenShield.js
@@ -713,13 +713,15 @@ const ScreenShield = new Lang.Class({
     _ensureUnlockDialog: function(onPrimary) {
         if (!this._dialog) {
             let constructor = Main.sessionMode.unlockDialog;
-            this._dialog = new constructor(this._lockDialogGroup);
-            if (!this._dialog) {
+            if (!constructor) {
                 // This session mode has no locking capabilities
                 this.unlock();
                 return;
             }
 
+            this._dialog = new constructor(this._lockDialogGroup);
+
+
             let time = global.get_current_time();
             this._dialog.connect('loaded', Lang.bind(this, function() {
                 if (!this._dialog.open(time, onPrimary)) {