Revert "tcp_abandon: no need to buffer pcb->local_port"

This reverts commit 1570dd8ad1.

Buffering pcb->local_port is needed because TCP_PCB_REMOVE_ACTIVE()
sets it to 0 via tcp_pcb_remove() (comment: "reset the local port
to prevent the pcb from being 'bound'").

Signed-off-by: Simon Goldschmidt <goldsimon@gmx.de>

src/core/tcp.c

@@ -583,6 +583,7 @@ tcp_abandon(struct tcp_pcb *pcb, int reset)
     tcp_free(pcb);
   } else {
     int send_rst = 0;
+    u16_t local_port = 0;
     enum tcp_state last_state;
     seqno = pcb->snd_nxt;
     ackno = pcb->rcv_nxt;
@@ -597,6 +598,7 @@ tcp_abandon(struct tcp_pcb *pcb, int reset)
       }
     } else {
       send_rst = reset;
+      local_port = pcb->local_port;
       TCP_PCB_REMOVE_ACTIVE(pcb);
     }
     if (pcb->unacked != NULL) {
@@ -613,7 +615,7 @@ tcp_abandon(struct tcp_pcb *pcb, int reset)
     tcp_backlog_accepted(pcb);
     if (send_rst) {
       LWIP_DEBUGF(TCP_RST_DEBUG, ("tcp_abandon: sending RST\n"));
-      tcp_rst(pcb, seqno, ackno, &pcb->local_ip, &pcb->remote_ip, pcb->local_port, pcb->remote_port);
+      tcp_rst(pcb, seqno, ackno, &pcb->local_ip, &pcb->remote_ip, local_port, pcb->remote_port);
     }
     last_state = pcb->state;
     tcp_free(pcb);