Avoid printing (null):-1 for STDIN in Nsock debug output

nsock/src/nsock_core.c

@@ -662,6 +662,7 @@ static int do_actual_read(struct npool *ms, struct nevent *nse) {
          * sockets, so peerlen is still sizeof(peer) and peer is junk. Instead,
          * only set this if it's not already set.
          */
+            && !IOD_PROPGET(iod, IOD_STDIN)
             && (iod->lastproto == IPPROTO_UDP || iod->peerlen == 0)
 #endif
         ) {