Added check for bad segments in fast retransmit. Attempted fix for tcp_out.c:1148 bug

ext/lwip/src/core/tcp_out.c

@@ -1145,6 +1145,11 @@ tcp_output(struct tcp_pcb *pcb)
         /* In the case of fast retransmit, the packet should not go to the tail
          * of the unacked queue, but rather somewhere before it. We need to check for
          * this case. -STJ Jul 27, 2004 */
+        if (seg == NULL || useg != NULL)
+        {
+          LWIP_DEBUGF(TCP_DEBUG, ("seg or useg was null, skipping this packet\n"));
+          return ERR_OK;
+        }
         if (TCP_SEQ_LT(lwip_ntohl(seg->tcphdr->seqno), lwip_ntohl(useg->tcphdr->seqno))) {
           /* add segment to before tail of unacked list, keeping the list sorted */
           struct tcp_seg **cur_seg = &(pcb->unacked);