managed_bytes: Make copy assignment exception-safe
managed_bytes: Make copy assignment exception-safe

raphaelsc/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,raphaelsc/scylla,avikivity/scylla,kjniemi/scylla,duarten/scylla,raphaelsc/scylla,scylladb/scylla,kjniemi/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,kjniemi/scylla