gdm: clear the messages queue when the user answers a prompt

the messages were being shown even when the user entered the
right information they were asked for.

https://bugzilla.gnome.org/show_bug.cgi?id=702458

js/gdm/util.js

@@ -164,6 +164,7 @@ const ShellUserVerifier = new Lang.Class({
 
     answerQuery: function(serviceName, answer) {
         if (!this._userVerifier.hasPendingMessages) {
+            this._clearMessageQueue();
             this._userVerifier.call_answer_query(serviceName, answer, this._cancellable, null);
         } else {
             let signalId = this._userVerifier.connect('no-more-messages',