fix: usr: Fix 'rndc modzone' issue with non-existing zones

The :iscman:`named` process could terminate unexpectedly or become
subject to undefined behavior when issued an :option:`rndc modzone`
operation for a non-existing zone. This has been fixed.

Closes #5848

Merge branch '5848-do_modzone-unlock-bug-fix' into 'main'

See merge request isc-projects/bind9!11844