pywrap: Progress callback proxy check for Py_None.
pywrap: Progress callback proxy check for Py_None.

If a wrapped function is called with callback parameter
set to None, i.e.

  callback = None
  loaders.Load("image_path/image", callback);

The progress callback fails because Py_None is not executable.

Fix this by checking for Py_None and skipping the callback if passed.

Signed-off-by: Cyril Hrubis <b1e90efd3b808b8e0bcf2a9a55fd8b9a779bfd81@ucw.cz>

gfxprim/gfxprim,gfxprim/gfxprim,gfxprim/gfxprim,gfxprim/gfxprim,gfxprim/gfxprim