improve S curve param check

Check if the S curve parameters that are sent to the controller are zero. The user might have set a value slightly above 0 but it might get truncated to 0 later. If that happens the following error occurs:
```
2014/11/18 13:24:25.571 p6kController::lowLevelWriteRead: ERROR: Command 1A0.00 returned an error: INVALID DATA-FIELD 1
```