Change actually_filter_streams to only run if subs overlay is open.

This change makes it such that the stream filtering operation will only
run if the subscription overlay is visible, preventing any issues with
the lack of existence of elements or processing something that users
won’t be able to see.

Fixes #3388.

static/js/subs.js

@@ -698,9 +698,11 @@ function ajaxSubscribe(stream) {
         url: "/json/users/me/subscriptions",
         data: {subscriptions: JSON.stringify([{name: stream}]) },
         success: function (resp, statusText, xhr) {
-            $("#create_stream_name").val("");
+            if (subs.is_open) {
+                $("#create_stream_name").val("");
 
-            actually_filter_streams();
+                actually_filter_streams();
+            }
 
             var res = JSON.parse(xhr.responseText);
             if (!$.isEmptyObject(res.already_subscribed)) {