Fix crash in verifyUserRelations

In the present implementation `verifyUserRelations(Model)` assumes that `Model.relations.accessTokens` is always set, and as a result may crash when trying to access `polymorphic` property of that relation. It seems the intention is to check whether the the following conditions are met: 1. a model has a hasMany accessTokens relation 2. that relation is polymorphic This commit fixes the problem by accounting for the case where the accessTokens relation was not correctly set up.

Author: ryanxwelch

lib/application.js

@@ -497,8 +497,10 @@ app._verifyAuthModelRelations = function() {
  function verifyUserRelations(Model) {
  const hasManyTokens = Model.relations && Model.relations.accessTokens;
 
+ const relationsConfig = Model.settings.relations || {};
+ const hasPolyMorphicTokens = (relationsConfig.accessTokens || {}).polymorphic;
  // display a temp warning message for users using multiple users config
- if (hasManyTokens.polymorphic) {
+ if (hasPolyMorphicTokens) {
  console.warn(
  'The app configuration follows the multiple user models setup ' +
  'as described in http://ibm.biz/setup-loopback-auth',
@@ -508,7 +510,6 @@ app._verifyAuthModelRelations = function() {
 
  if (hasManyTokens) return;
 
- const relationsConfig = Model.settings.relations || {};
  const accessTokenName = (relationsConfig.accessTokens || {}).model;
  if (accessTokenName) {
  console.warn(