[CVE-2025-40777] sec: usr: Fix a possible assertion failure when using the 'stale...

In specific circumstances the :iscman:`named` resolver process could
terminate unexpectedly when stale answers were enabled and the
``stale-answer-client-timeout 0`` configuration option was used.
This has been fixed.

See isc-projects/bind9#5372

Merge branch '5372-security-serve-stale-crash-on-insist-unreachable' into 'v9.21.10-release'

See merge request isc-private/bind9!808