fix: dev: handle KSR files with DNSKEY records before any header

A DNSKEY record appearing before the first ';; KeySigningRequest'
header in a KSR file made dnssec-ksr abort on an internal assertion
instead of producing a structured error, killing pipelines that
fed it crafted or corrupted input.  The tool now exits with a
fatal error naming the file and line.

Closes #5914

Merge branch '5914-dnssec-ksr-rdatalist-null-insist' into 'main'

See merge request isc-projects/bind9!11916