Revert 64359a3: "Fixed typo $length > $bufLength"

Because it drops:
```
unpack(): Type N: not enough input, need 4, have 3 (SmppClient:736)
extract() expects parameter 1 to be array, boolean given (SmppClient:736)
socket_recv(): unable to read from socket [11]: Resource temporarily unavailable (SocketTransport:332)
Uncaught exception 'RuntimeException' with message 'Could not read PDU body' (SmppClient:741)
```
@Dexus: This fix was proposed by @ColmMcBarron at 64359a3 but it does not work for me at all.