Reuse class hint in the status-bar.

Reuse class hint in the status-bar.

The window's class hint is already retrieved by the manage_window
function, which is called when a new application is spawned. Therefore,
use it when displaying the application's class and name, instead of
fetching them again with XGetClassHint(3).

Besides, its error checking was incorrect, since it also returns zero on
error.