rgw/lua: fix CopyFrom crash

[yuvalif]

Fixes: https://tracker.ceph.com/issues/59381

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[yuvalif]

passing teuthology: http://pulpito.front.sepia.ceph.com/yuvalif-2023-04-17_17:32:17-rgw:notifications-wip-yuval-test-distro-default-smithi/
(ran just the multisite tests, as this is a small change and there is no lua suite anyway)