block: remove redundant check before g_slist_find()

An empty GSList is represented by a NULL pointer, therefore it's a
perfectly valid argument for g_slist_find() and there's no need to
make any additional check.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: 1435583533-5758-1-git-send-email-berto@igalia.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
