sockets: Fix free-select-locked after udp close

ESP_LWIP supports closing socket while waiting. This fixes potential
races.
Orig: fix bug for after udp close select_waiting assert (dd3b3017)

Ref IDF-4794

src/api/sockets.c

@@ -565,6 +565,9 @@ free_socket_locked(struct lwip_sock *sock, int is_tcp, struct netconn **conn,
 
   *lastdata = sock->lastdata;
   sock->lastdata.pbuf = NULL;
+#if ESP_LWIP
+  sock->select_waiting = 0;
+#endif /* ESP_LWIP */
   *conn = sock->conn;
   sock->conn = NULL;
   return 1;