Don't throw an exception if someone edits a very old message

Previously, if you didn't have a message that someone else had edited
in your message list, you would get an exception because we weren't
checking whether we actually had the message or not.

(imported from commit 33a5c6e7fe95b5397a32df5c7b5f6714d71e1e5f)

zephyr/static/js/zephyr.js

@@ -719,6 +719,10 @@ function maybe_add_narrowed_messages(messages, msg_list) {
 function update_messages(events) {
     $.each(events, function (idx, event) {
         var msg = all_msg_list.get(event.message_id);
+        if (msg === undefined) {
+            return;
+        }
+
         if (event.rendered_content !== undefined) {
             msg.content = event.rendered_content;
         }