auth/standard_role_manager: Fix life-time bug
auth/standard_role_manager: Fix life-time bug

It worked most of the time, but changes in other areas of the code must
have triggered the conditions necessary to make it fail.

avikivity/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,duarten/scylla