Clean up container references

Calling lxc_container_put() multiple times can cause a segv:

    struct lxc_container *c = lxc_container_new("erlxc", NULL);
    lxc_container_put(c);
    lxc_container_put(c);

lxc_container_put() calls lxc_container_free() which deallocates the
lxc_container structure.