xwayland: Fix crash when run with no input device

Starting an xterm with no input device led to a crash
because weston_wm_pick_seat() was returning garbage and
weston_wm_selection_init() was trying to use the garbage.

Signed-off-by: Tom Hochstein <tom.hochstein@nxp.com>
Reviewed-by: Giulio Camuffo <giuliocamuffo@gmail.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>

xwayland/selection.c

@@ -709,6 +709,8 @@ weston_wm_selection_init(struct weston_wm *wm)
 					  wm->atom.clipboard, mask);
 
 	seat = weston_wm_pick_seat(wm);
+	if (seat == NULL)
+		return;
 	wm->selection_listener.notify = weston_wm_set_selection;
 	wl_signal_add(&seat->selection_signal, &wm->selection_listener);
 

xwayland/window-manager.c

@@ -1306,8 +1306,10 @@ weston_wm_handle_reparent_notify(struct weston_wm *wm, xcb_generic_event_t *even
 struct weston_seat *
 weston_wm_pick_seat(struct weston_wm *wm)
 {
-	return container_of(wm->server->compositor->seat_list.next,
-			    struct weston_seat, link);
+	struct wl_list *seats = wm->server->compositor->seat_list.next;
+	if (wl_list_empty(seats))
+		return NULL;
+	return container_of(seats, struct weston_seat, link);
 }
 
 static struct weston_seat *