[cogl] Fix return value of cogl.Texture.get_data
[cogl] Fix return value of cogl.Texture.get_data

The current implementation of pycogl_texture_get_data did not return
anything, as cogl_texture_get_data was called with a NULL pointer.

Signed-off-by: Bastian Winkler <6b369e490de120a99ec239807263a55ceec7b8a3@netbuz.org>

pmarti/pyclutter,GNOME/pyclutter,pmarti/pyclutter,pmarti/pyclutter,GNOME/pyclutter,pmarti/pyclutter