atexit: Fix a bug in credential cleanup
atexit: Fix a bug in credential cleanup

This bug occured at random so it was somewhat pesky.

Signed-off-by: Benoît Canet <ecd1f14f7c1c6dc7a40210bdcc3810e0107ecbc8@scylladb.com>

amoskong/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,amoskong/scylla-cluster-tests,scylladb/scylla-cluster-tests,amoskong/scylla-cluster-tests,amoskong/scylla-cluster-tests,amoskong/scylla-cluster-tests,scylladb/scylla-longevity-tests