Fix inconsistent tokens received by other servers

When use `coap-client` with -T command, the other server received inconsistent.
```
coap-client send token "aaa", the other server received "aab"
coap-client send token "obstoken", the other server received "obstokeo"
```

src/coap_session.c

@@ -1583,7 +1583,7 @@ void
 coap_session_new_token(coap_session_t *session, size_t *len,
  uint8_t *data) {
  *len = coap_encode_var_safe8(data,
- sizeof(session->tx_token), ++session->tx_token);
+ sizeof(session->tx_token), session->tx_token++);
 }
 
 uint16_t