Apply [patch #9287] Possible null-pointer dereference from Mikhail Lappo in a modified way

Moved the debug output to a location where we know that newpcb != NULL
Fixes possible NULL pointer dereferencing in debug message output

src/api/api_msg.c

@@ -471,8 +471,6 @@ accept_function(void *arg, struct tcp_pcb *newpcb, err_t err)
   struct netconn *newconn;
   struct netconn *conn = (struct netconn *)arg;
 
-  LWIP_DEBUGF(API_MSG_DEBUG, ("accept_function: newpcb->tate: %s\n", tcp_debug_state_str(newpcb->state)));
-
   if (conn == NULL) {
     return ERR_VAL;
   }
@@ -490,6 +488,8 @@ accept_function(void *arg, struct tcp_pcb *newpcb, err_t err)
     return ERR_VAL;
   }
 
+  LWIP_DEBUGF(API_MSG_DEBUG, ("accept_function: newpcb->tate: %s\n", tcp_debug_state_str(newpcb->state)));
+
   /* We have to set the callback here even though
    * the new socket is unknown. newconn->socket is marked as -1. */
   newconn = netconn_alloc(conn->type, conn->callback);