gdm: clear user verifier when finished with it

We only need the user verifier for the purpose of user verification.
Once it's complete we should clear it so it doesn't get in the way
later.

This fixes a bug introduced in commit 3c8c5a5570 that leads to the
user session crashing when the login screen is reactivated.

https://bugzilla.gnome.org/show_bug.cgi?id=753181

js/gdm/authPrompt.js

@@ -489,6 +489,7 @@ const AuthPrompt = new Lang.Class({
 
     finish: function(onComplete) {
         if (!this._userVerifier.hasPendingMessages) {
+            this._userVerifier.clear();
             onComplete();
             return;
         }
@@ -496,6 +497,7 @@ const AuthPrompt = new Lang.Class({
         let signalId = this._userVerifier.connect('no-more-messages',
                                                   Lang.bind(this, function() {
                                                       this._userVerifier.disconnect(signalId);
+                                                      this._userVerifier.clear();
                                                       onComplete();
                                                   }));
     },