[9.20] fix: dev: Fix data race on fctx->vresult in validated()

Move the write to fctx->vresult after LOCK(&fctx->lock).  The field was
being set before acquiring the lock, but dns_resolver_logfetch() reads
it under the same lock from another thread.

Backport of MR !11717

Merge branch 'backport-ondrej/fix-data-race-on-fctx-result-in-validated-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11721