fixed bug #33871: rejecting TCP_EVENT_RECV() for the last packet including FIN can lose data

src/include/lwip/pbuf.h

@@ -69,6 +69,8 @@ typedef enum {
 #define PBUF_FLAG_IS_CUSTOM 0x02U
 /** indicates this pbuf is UDP multicast to be looped back */
 #define PBUF_FLAG_MCASTLOOP 0x04U
+/** indicates this pbuf includes a TCP FIN flag */
+#define PBUF_FLAG_TCP_FIN   0x20U
 
 struct pbuf {
   /** next pbuf in singly linked pbuf chain */