Check for NULL before dereferencing qctx->rpz_st

Check for NULL before dereferencing qctx->rpz_st

Commit 9ffb4a7ba11fae64a6ce2dd6390cd334372b7ab7 causes Clang Static
Analyzer to flag a potential NULL dereference in query_nxdomain():

    query.c:9394:26: warning: Dereference of null pointer [core.NullDereference]
            if (!qctx->nxrewrite || qctx->rpz_st->m.rpz->addsoa) {
                                    ^~~~~~~~~~~~~~~~~~~
    1 warning generated.

The warning above is for qctx->rpz_st potentially being a NULL pointer
when query_nxdomain() is called from query_resume().  This is a false
positive because none of the database lookup result codes currently
causing query_nxdomain() to be called (DNS_R_EMPTYWILD, DNS_R_NXDOMAIN)
can be returned by a database lookup following a recursive resolution
attempt.  Add a NULL check nevertheless in order to future-proof the
code and silence Clang Static Analyzer.

(cherry picked from commit 07592d1315412c38c978e8d009aace5d0f5bef93)
(cherry picked from commit a4547a109324fff1bdd21032c5c7d8fdeb0e4ad8)