sdl_image: fix filename strings freed too early
sdl_image: fix filename strings freed too early

In ReadXPMFromArray() and SavePNG(), the passed filename parameter are
converted to CString but then freed immediately using C.free().

Signed-off-by: Lilis Iskandar <f9acb790f2c75b127492316321ece0121b8eed9b@veandco.org>

veandco/go-sdl2,veandco/go-sdl2