Fix safe event check.

Author: davidlehn

lib/events.js

@@ -103,7 +103,8 @@ function _handle({event, handlers}) {
 // safe handler that rejects unsafe warning conditions
 api.safeModeEventHandler = function safeModeEventHandler({event, next}) {
   // fail on all unsafe warnings
-  if(event.level === 'warning' && event.tags.includes('unsafe')) {
+  if(event.level === 'warning' &&
+    event.tags && event.tags.includes('unsafe')) {
     throw new JsonLdError(
       'Safe mode violation.',
       'jsonld.SafeModeViolationEvent',