[9.18] fix: dev: Tidy up the cleanup path in check_signer()

When check_signer() processed a DNSKEY whose public-key data could not
be parsed, the early return on the parse error skipped the cleanup of
the cloned signature rdataset.  In every code path that currently
reaches this function the cloned rdataset holds no resources, so no
memory was actually leaked, but the cleanup is restructured so the
parse and the iteration cannot diverge again.

Closes #5869

Backport of MR !11957

Merge branch 'backport-5869-fix-memory-leak-in-check_signer-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!11960