authPrompt: Clear _queryingService on verification failure

A conversation is finished after failing, and we are expecting a new
one to be started shortly after. However if we encounter an existing
reference to a previously set _queryingService, we will clear the
password entry, which might already contain a partially typed password
at that point. The behavior does make sense in the case of conflicting
conversations, but in the failure case it is both unexpected and
annoying, so clear _queryingService early to prevent this.

https://bugzilla.gnome.org/show_bug.cgi?id=708186

js/gdm/authPrompt.js

@@ -244,6 +244,7 @@ const AuthPrompt = new Lang.Class({
     },
 
     _onVerificationFailed: function() {
+        this._queryingService = null;
         this.clear();
 
         this.updateSensitivity(true);