tcp: improve debug message a little

(cherry picked from commit ed561a578b3283593c38a206d4a9b735e9d84758)

src/core/tcp_in.c

@@ -852,8 +852,9 @@ tcp_process(struct tcp_pcb *pcb)
   /* Do different things depending on the TCP state. */
   switch (pcb->state) {
     case SYN_SENT:
-      LWIP_DEBUGF(TCP_INPUT_DEBUG, ("SYN-SENT: ackno %"U32_F" pcb->snd_nxt %"U32_F" unacked %"U32_F"\n", ackno,
-                                    pcb->snd_nxt, (pcb->unacked != NULL)? lwip_ntohl(pcb->unacked->tcphdr->seqno) : 0));
+      LWIP_DEBUGF(TCP_INPUT_DEBUG, ("SYN-SENT: ackno %"U32_F" pcb->snd_nxt %"U32_F" unacked %s %"U32_F"\n",
+                                    ackno, pcb->snd_nxt, pcb->unacked ? "" : " empty:",
+                                    pcb->unacked ? lwip_ntohl(pcb->unacked->tcphdr->seqno) : 0));
       /* received SYN ACK with expected sequence number? */
       if ((flags & TCP_ACK) && (flags & TCP_SYN)
           && (ackno == pcb->lastack + 1)) {