xwm: Destroy xwm on hangup or error

If Xwayland is restarted, the ready handler assumes there is no xwm instance.
This means all of xwm was leaked on Xwayland restart. This caused compositors
to consume all cpu resources, where time is spent dispatching. Now we destroy
xwm if we get an event mask containing WL_EVENT_HANGUP or WL_EVENT_ERROR.

xwayland/xwm.c

@@ -1312,6 +1312,11 @@ static int x11_event_handler(int fd, uint32_t mask, void *data) {
 	xcb_generic_event_t *event;
 	struct wlr_xwm *xwm = data;
 
+	if ((mask & WL_EVENT_HANGUP) || (mask & WL_EVENT_ERROR)) {
+		xwm_destroy(xwm);
+		return 0;
+	}
+
 	while ((event = xcb_poll_for_event(xwm->xcb_conn))) {
 		count++;
 
@@ -1493,6 +1498,7 @@ void xwm_destroy(struct wlr_xwm *xwm) {
 	wl_list_remove(&xwm->compositor_destroy.link);
 	xcb_disconnect(xwm->xcb_conn);
 
+	xwm->xwayland->xwm = NULL;
 	free(xwm);
 }