fixed Lwip bug: Assert on TCP netconn_write with sndtimeout set

ref: http://savannah.nongnu.org/bugs/?38219

components/net/lwip-1.4.1/src/api/api_msg.c

@@ -1235,6 +1235,7 @@ do_writemore(struct netconn *conn)
       /* partial write */
       err = ERR_OK;
       conn->current_msg->msg.w.len = conn->write_offset;
+      conn->write_offset = 0;
     }
   } else
 #endif /* LWIP_SO_SNDTIMEO */

components/net/lwip-head/src/api/api_msg.c

@@ -1261,6 +1261,7 @@ lwip_netconn_do_writemore(struct netconn *conn)
       /* partial write */
       err = ERR_OK;
       conn->current_msg->msg.w.len = conn->write_offset;
+      conn->write_offset = 0;
     }
   } else
 #endif /* LWIP_SO_SNDTIMEO */