Caused by c7aa0f9d743ee9cafe91b4a51d9f1ecc5e437d71.
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.