A bug that violates the specification of the length field

[jyjsunny]

Hi, I found a bug that violates the specification that the length field should adhere to.

Steps to reproduce(after installation):

1. Terminal 1 - Server

cd libmodbus/tests
./unit-test-server

2. Terminal 2 - Client First, send one valid input: b'\x00\x01\x00\x00\x00\x06\xff\x01\x01\x30\x00\x06 Second, send invalid inputs that contain incorrect length field values: b'\x00\x01\x00\x00\x00\x00\xff\x01\x01\x30\x00\x06 b'\x00\x01\x00\x00\x10\x00\xff\x01\x01\x30\x00\x06 3）Observe the server's outputs; we can see that they received the same positive response. b'\x00\x01\x00\x00\x00\x04\xff\x01\x01\x00

The Modbus/TCP protocol specification that the bug violates is as follows: