Clear qctx->zversion when clearing qctx->zrdataset et al in
lib/ns/query.c:qctx_freedata. The uncleared pointer could lead to
an assertion failure if zone data needed to be re-saved which could
happen with stale data support enabled.
(cherry picked from commit 179fb3532ab8d4898ab070b2db54c0ce872ef709)
9cfd20cd90