[9.20] fix: usr: Fix a crash triggered by rndc modzone on zone that already existed...

Calling `rndc modzone` didn't work properly for a zone hat was configured in
the configuration file. It could crash if BIND 9 was built without LMDB or if
there was already an NZF file for the zone. In addition, `rndc modzone` failed
in subsequent attempts. These problems are now fixed.

Closes #5826

Merge branch '5826-fix-modzone-issues-ytatuya' into 'bind-9.20'

See merge request isc-projects/bind9!11743