[CVE-2026-3104] sec: usr: Fix memory leaks in code preparing DNSSEC proofs of non...

An attacker controlling a DNSSEC-signed zone could trigger a memory leak
in the logic preparing DNSSEC proofs of non-existence, by creating more
than :any:`max-records-per-type` RRSIGs for NSEC records. These memory
leaks have been fixed.

ISC would like to thank Vitaly Simonovich for bringing this
vulnerability to our attention.

Closes isc-projects/bind9#5742

Merge branch '5742-fix-memory-leak-in-addnoqname-and-addclosest' into 'v9.21.20-release'

See merge request isc-private/bind9!913