[9.18] fix: usr: Prevent a crash when using both dns64 and filter-aaaa

An assertion failure could be triggered if both `dns64` and the `filter-aaaa` plugin were in use simultaneously. This happened if the plugin triggered a second recursion process, which then attempted to store DNS64 state information in a pointer that had already been set by the original recursion process. This has been fixed.

Closes #5854

Backport of MR !11949

Merge branch 'backport-5854-dns64-aaaaok-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!11968