[CVE-2026-5947] sec: usr: Fix crash in resolver when SIG(0)-signed responses are...

A resolver could crash when handling a SIG(0)-signed response if the
matching client query was cancelled while signature verification was
still in progress — for example, when the recursive-clients quota
was exhausted. This has been fixed.

Closes isc-projects/bind9#5819

Merge branch '5819-fix-heap-use-after-free-in-resquery_response_continue' into 'security-main'

See merge request isc-private/bind9!964