[9.18] fix: dev: Fix a logic error in cache_name()

A change in 6aba56ae8 (checking whether a rejected RRset was identical
to the data it would have replaced, so that we could still cache a
signature) inadvertently introduced cases where processing of a
response would continue when previously it would have been skipped.

Closes #5197

Backport of MR !10157

Merge branch 'backport-5197-cache_name-logic-error-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10159