optimize decompress jpg data

it may be unnecessary copy jpg bytes one by one when decompress jpg data
Why need to add this line?
To prevent been destroyed in 

```c++
if (row_pointer[0] != nullptr)
    {
        free(row_pointer[0]);
    };
```
?

If so, i think we can remove the codes.

And please fix warnings. After this modification, `i` is not used.

Thanks.