lightbox: Don't blow up for messages not in the message store.

This should somewhat reduce the gravity of the failure mode for cases
where the message the user clicked cannot be found (which would be a
significant bug on its own merit in any case).

static/js/lightbox.js

@@ -138,8 +138,9 @@ exports.open = function (image, options) {
             const message = message_store.get(zid);
             if (message === undefined) {
                 blueslip.error("Lightbox for unknown message " + $message.attr("zid"));
+            } else {
+                sender_full_name = message.sender_full_name;
             }
-            sender_full_name = message.sender_full_name;
         }
         payload = {
             user: sender_full_name,