clipboard: remove the weston_seat destruction listener on destroy

Prior to freeing the memory in which the link node for the signal is
emedded we should remove the link node from the list to prevent the list
from being corrupted.

https://bugs.freedesktop.org/show_bug.cgi?id=67231

src/clipboard.c

@@ -256,6 +256,7 @@ clipboard_destroy(struct wl_listener *listener, void *data)
 		container_of(listener, struct clipboard, destroy_listener);
 
 	wl_list_remove(&clipboard->selection_listener.link);
+	wl_list_remove(&clipboard->destroy_listener.link);
 
 	free(clipboard);
 }