Fix null deref in wlr_libinput_backend_destroy
Fix null deref in wlr_libinput_backend_destroy

If input_event is null (e.g. if backend_start has not been called yet)
wl_event_source_remove will result in a null deref.

ascent12/wlroots,swaywm/wlroots,ascent12/wlroots,SirCmpwn/wlroots,swaywm/wlroots