Blender Git Loki

Bugfix #18266

Mipmap creation for render crashed, in this case:
- use Curves tool on an Image, which is UV mapped on object
- Save the image to disk, under new name
- Render (F12)

This fix is only for the crash, there's something not well coded
for Curves tool, how it manages float buffers. That's for 2.5.