[9.20] sec: usr: Fix crash when reconfiguring zone update policy during active updates

Fixed a crash that could occur when running rndc reconfig to change a zone's update policy (e.g., from allow-update to update-policy) while DNS UPDATE requests were being processed for that zone.

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

Fixes #5817

Backport of MR !11707

Merge branch 'backport-5817-fix-crash-via-SSU-table-desynchronization-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11738