server: Avoid NULL pointer dereference in make_status_items().

server: Avoid NULL pointer dereference in make_status_items().

The previous patch (Update status items on blob events) modified the
aft event handler to call make_status_items() on certain blob events
because the event could have rendered the current status information
stale.

However, if no audio file is open at the time the event occurs,
make_status_items() triggers a segfault because the ->path and ->hash
members of status_item_ls_data are NULL in this case.

This patch avoids the problem by returning early from
make_status_items() if ->path is NULL.