[9.20] fix: usr: Fix an assertion failure in validate_dnskey_dsset_done()

Under rare circumstances, named could terminate unexpectedly
when validating a DNSKEY resource record if the validation
was canceled in the meantime. This has been fixed.

Closes isc-projects/bind9#4911

Backport of isc-private/bind9!731

Merge branch 'backport-4911-assertion-failure-in-validate_dnskey_dsset_done-9.20' into 'v9.20.2-release'

See merge request isc-private/bind9!734