Don't consider dialog as still running if wait() fails

I'm not sure if we can trust the state of things if the wait() call
actually fails (as in, the call itself returns an error), so let's just
let the dialog die and hope that if there's more user activity it'll
start up again properly.

locker/src/screensaver.rs

@@ -305,7 +305,7 @@ impl<'a> Screensaver<'a> {
             match unlock_dialog.child.try_wait() {
                 Err(err) => {
                     warn!("Failed to check unlock dialog's status: {}", err);
-                    self.unlock_dialog = Some(unlock_dialog);
+                    self.hide_cursor();
                 },
                 Ok(Some(status)) => {
                     let ok = match DialogExitStatus::try_from(status) {