delete: Don't wrongly set window as not alive when pings are disabled

meta_display_ping_window() does nothing when check-alive-timeout is set to 0, but meta_window_check_alive_on_event() was relying on it to reset the events_during_ping. Without this events_during_ping was just counting up until the threshold was reached and the window was marked as not alive, preventing further pointer events from being sent to the window.

Fix this by not doing anything in meta_window_check_alive_on_event() if check-alive-timeout is 0, similar to meta_display_ping_window().

Closes: #3142 (closed)