[9.20] fix: dev: Fix a stack use-after-free in qpzone

In previous_closest_nsec(), a new qpreader was opened to search the NSEC
tree. It was possible for that to be used to update a QP iterator object
owned by the caller, and then be destroyed when the function returned.

This has been addressed by having the caller open the NSEC qpreader
instead.

Closes #5942

Merge branch '5942-qpiter-fix-bind-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11956