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 qpreader object isn't necessary anymore; since namespaces were
added to the QP trie in commit 15653c54a0, we can now just reuse the
existing reader for the main tree.

Closes #5942

Merge branch '5942-qpiter-fix' into 'main'

See merge request isc-projects/bind9!11955