Allow an invalid session to logout from logedin state

src/octoprint/static/js/app/viewmodels/loginstate.js

@@ -104,6 +104,11 @@ $(function() {
                 success: function(response) {
                     new PNotify({title: gettext("Logout successful"), text: gettext("You are now logged out"), type: "success"});
                     self.fromResponse(response);
+                },
+                error: function(error) {
+                    if (error && error.status === 401) {
+                         self.fromResponse(false);
+                    }
                 }
             })
         };