gdmUtil: Fix auth prompt drawing over user list

If the user fails to enter their password then hits escape, we
jump back to the user list, then ask again for a password in a
garbled screen. this commit fixes that by skipping a retry if
the operation is cancelled.

https://bugzilla.gnome.org/show_bug.cgi?id=784361

js/gdm/util.js

@@ -540,6 +540,7 @@ const ShellUserVerifier = new Lang.Class({
                 let signalId = this.connect('no-more-messages',
                                             Lang.bind(this, function() {
                                                 this.disconnect(signalId);
+                                                if (this._cancellable && !this._cancellable.is_cancelled())
                                                     this._retry();
                                             }));
             }