[9.20] fix: dev: Fix memory leak in dns_catz_options_setdefault() for zonedir

When defaults->zonedir is set, opts->zonedir is unconditionally
overwritten without freeing the previous value. This leaks memory
on every catalog zone update when zonedir defaults are configured.

Free the existing opts->zonedir before replacing it.

Backport of MR !11660

Merge branch 'backport-ondrej/fix-memory-leak-in-dns_catz_options_setdefault-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11685