bug fix: memory leak in STRING_DATA handler.

The board loses response after sending STRING_DATA message for some dozen times.

This is because ```buffer``` in  ```FirmataClass::processSysexMessage``` is left allocated: 
```c
char *buffer = (char*)malloc(bufferLength * sizeof(char));
```

So I deallocated it after the stringCallback call.
Now, we can keep control even after sending STRING_DATA messages for hundreds of times.