Fix NPE in updateuser code

Fixes https://sentry.io/coreos/frontend-production/issues/364782088/

static/js/pages/update-user.js

@@ -60,7 +60,7 @@
     };
 
     $scope.hasPrompt = function(user, prompt_name) {
-      if (!user.prompts) {
+      if (!user || !user.prompts) {
         return false;
       }