resolved: refuse OPT RRs in incoming packets that are not in the additional section

We later rely that the DnsAnswer object contains all RRs from the
original packet, at least when it comes to the answer and authorization
sections, hence we better make sure we don#t silently end up removing an
OPT RR from these two sections.

src/resolve/resolved-dns-packet.c

@@ -1993,8 +1993,18 @@ int dns_packet_extract(DnsPacket *p) {
                                 goto finish;
 
                         if (rr->key->type == DNS_TYPE_OPT) {
-                                if (p->opt)
-                                        return -EBADMSG;
+
+                                /* The OPT RR is only valid in the Additional section */
+                                if (i < DNS_PACKET_ANCOUNT(p) + DNS_PACKET_NSCOUNT(p)) {
+                                        r = -EBADMSG;
+                                        goto finish;
+                                }
+
+                                /* Two OPT RRs? */
+                                if (p->opt) {
+                                        r = -EBADMSG;
+                                        goto finish;
+                                }
 
                                 p->opt = dns_resource_record_ref(rr);
                         } else {