In dns_qpiter_{prev,next}, defer dereference_iter_node call

In dns_qpiter_{prev,next}, defer dereference_iter_node call

dns_qpiter_{prev,next} requires the current iterator node to still be
valid which might not always the case after dereference_iter_node was
called.  Currently, this is ensured via closeversion() mechanism, but it
is not guaranteed to be true in the future.

Move the call to dereference_iter_node to after the dns_qpiter_prev()
and dns_qpiter_next() to prevent a possible use-after-free of the
current iterator node.

(cherry picked from commit 9914bd383ecd3f1dc1ed0a98171c79f0614988b3)