[9.20] fix: usr: Fix the assertion failure in the selfsigned DNSKEY handling

The selfsigned_dnskey() function can now return all the return codes
that dns_dnssec_keyfromrdata() can return and this would cause an
assertion failure as we were not expecting new isc_result_t codes.

Backport of !865

Closes isc-projects/bind9#5343

Merge branch 'ondrej/security-fix-crash-in-selfsigned-key-handling-9.20' into 'v9.20.15-release'

See merge request isc-private/bind9!866