message_list: Fix handling of unknown email in mentions.

If a user has an old mention and has since been renamed, there's
really nothing for us to do to render it; we should just return as
though we have no data.

static/js/message_list_view.js

@@ -32,7 +32,11 @@ function get_user_id_for_mention_button(elem) {
 
     if (email) {
         // Will return undefined if there's no match
-        return people.get_by_email(email).user_id;
+        var user = people.get_by_email(email);
+        if (user) {
+            return user.user_id;
+        }
+        return;
     }
     return;
 }