Review a calls to C.CString and pair them with C.free where necessary.

[jabolopes]

Hi,

This is part of issue https://github.com/veandco/go-sdl2/issues/582

I went over all calls to C.CString and checked whether they are paired with C.free.

The patch series contains 3 patches.

• The first patches fixes 1 call site that is using C.SDL_free instead of C.free.
• The second patch adds C.free calls to C.CString calls that do not have a free.
• The third patch is fixes a call site that handles C strings that may be potentially nil.

Feedback is welcome!

Thanks, Jose