shell.c: Not all seats have pointers

Don't look up the pointer focus in ping_timeout_handler() if the
seat doesn't have a pointer.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=73490

desktop-shell/shell.c

@@ -1786,7 +1786,7 @@ ping_timeout_handler(void *data)
 	shsurf->unresponsive = 1;
 
 	wl_list_for_each(seat, &shsurf->surface->compositor->seat_list, link)
-		if (seat->pointer->focus &&
+		if (seat->pointer && seat->pointer->focus &&
 		    seat->pointer->focus->surface == shsurf->surface)
 			set_busy_cursor(shsurf, seat->pointer);