Fix assertion failure when server is under load and root zone is not yet loaded.

When we're coming back from recursion fetch_callback does not accept DNS_R_NXDOMAIN as an rcode - query_gotanswer calls query_nxdomain in which an assertion fails on qctx->is_zone. Yet, under some circumstances, qname minimization will return an DNS_R_NXDOMAIN - when root zone mirror is not yet loaded. The fix changes the DNS_R_NXDOMAIN answer to DNS_R_SERVFAIL.

Closes #1862 (closed)