[9.18] sec: usr: Fix an issue when some specific queries could remain unanswered...

When :iscman:`named` was running with stale answers enabled and with the
``stale-answer-client-timeout 0`` configuration option, in certain
situations it was possible that some queries could remain unanswered.
This has been fixed.

Closes isc-projects/bind9#5383

Merge branch '5383-security-stale-answer-client-timeout-0-no-response-fix' into 'v9.18.38-release'

See merge request isc-private/bind9!813