issue: 4607536 Update last_unacked in syn sent state

after progressing pcb->unacked we need to check
if it becomes null then we need to nullify pcb->last_unacked

Signed-off-by: Bashar Abdelgafer <[email protected]>

Author: BasharRadya

src/vma/lwip/tcp_in.c

@@ -575,6 +575,7 @@ tcp_process(struct tcp_pcb *pcb, tcp_in_data* in_data)
       /* If there's nothing left to acknowledge, stop the retransmit
          timer, otherwise reset it to start again */
       if(pcb->unacked == NULL) {
+        pcb->last_unacked = NULL;
         pcb->rtime = -1;
       } else {
         pcb->rtime = 0;