gdm: unconditionally cancel auth user verifier on reset

We currently only cancel the user verifier on reset if
verifying, but that means we don't properly cancel it when
asking for a username at the Not Listed screen.

The object already handles getting called when there is
nothing to cancel, so just cancel it unconditionally.

https://bugzilla.gnome.org/show_bug.cgi?id=752438

js/gdm/authPrompt.js

@@ -433,7 +433,7 @@ const AuthPrompt = new Lang.Class({
         this.verificationStatus = AuthPromptStatus.NOT_VERIFYING;
         this.cancelButton.reactive = true;
 
-        if (oldStatus == AuthPromptStatus.VERIFYING)
+        if (this._userVerifier)
             this._userVerifier.cancel();
 
         this._queryingService = null;