Caused by c7aa0f9d74.
Since above commit, BKE_image_user_frame_calc requires an image (not
just the iuser) to get the framenumber.
Cycles used to call this with NULL image (in `image_user_file_path` and
`image_user_frame_number`), now pass the image as well.
Maniphest Tasks: T79111
Differential Revision: https://developer.blender.org/D8439