avoid uninitialised free of ldns_res

If an invalid rdclass was passed to getrrsetbyname() then
this would execute a free on an uninitialised pointer.
OpenSSH only ever calls this with a fixed and valid rdclass.

Reported by Joshua Rogers

openbsd-compat/getrrsetbyname-ldns.c

@@ -69,7 +69,7 @@ getrrsetbyname(const char *hostname, unsigned int rdclass,
 	struct rrsetinfo *rrset = NULL;
 	struct rdatainfo *rdata;
 	size_t len;
-	ldns_resolver *ldns_res;
+	ldns_resolver *ldns_res = NULL;
 	ldns_rdf *domain = NULL;
 	ldns_pkt *pkt = NULL;
 	ldns_rr_list *rrsigs = NULL, *rrdata = NULL;