[9.18] fix: usr: Prevent spurious validation failures

Under rare circumstances, validation could fail if multiple clients simultaneously iterated the same set of signatures.

References #3014

Backport of MR !5578

Merge branch 'backport-3014-validator-c-check_signer-fails-to-call-dns_rdataset_clone-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10816