dns (tcp) - fix coverity cid 1374307

CID 1374307 (#1 of 1): Dereference before null check (REVERSE_INULL)
check_after_deref: Null-checking dns_state suggests that it may be null,
but it has already been dereferenced on all paths leading to the check.
317    if (dns_state != NULL && f != NULL) {
318        dns_state->last_resp = f->lastts;
319    }

src/app-layer-dns-udp.c

@@ -314,7 +314,7 @@ static int DNSUDPResponseParse(Flow *f, void *dstate,
 
         tx->replied = 1;
     }
-    if (dns_state != NULL && f != NULL) {
+    if (f != NULL) {
         dns_state->last_resp = f->lastts;
     }
     SCReturnInt(1);