unread_ops: Harden against bugs involving recent topics.
unread_ops: Harden against bugs involving recent topics.

In theory, this function should never be called when Recent Topics is
visible. But if it somehow is, ensure that we don't access
message_lists.current without first checking whether it's visible.

andersk/zulip,rht/zulip,zulip/zulip,zulip/zulip,andersk/zulip,andersk/zulip,rht/zulip,rht/zulip,zulip/zulip,rht/zulip,zulip/zulip,rht/zulip,rht/zulip,zulip/zulip,rht/zulip,zulip/zulip,andersk/zulip,andersk/zulip,andersk/zulip,zulip/zulip,andersk/zulip