Don't error out on a name change for a stream you're not on.

(imported from commit 8438087b3b6c66f41b9d9152661a93f01a086b28)

static/js/subs.js

@@ -120,6 +120,11 @@ function update_stream_notifications(sub, value) {
 }
 
 function update_stream_name(sub, new_name) {
+    if (sub === undefined) {
+        // This isn't a stream we know about, so ignore it.
+        return;
+    }
+
     // Rename the stream internally.
     var old_name = sub.name;
     stream_data.delete_sub(old_name);