[9.20] fix: dev: Validating ADB fetches could cause a crash in import_rdataset()

Previously, in some cases, the resolver could return rdatasets of type CNAME or DNAME without the result code being set to `DNS_R_CNAME` or `DNS_R_DNAME`. This could trigger an assertion failure in the ADB. The resolver error has been fixed.

Closes #5201

Backport of MR !10172

Backport of MR !10178

Merge branch 'backport-5201-adb-cname-error-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10329