Possible fix for T42567

Short description of the error
The autokey feature failed to set keyframes for all array values of Color Wheels (noticeable e.g. in Color Balance Node or Color Balance Modifier). This happend due to a wrong use of FCurve->array_index.

I'm not 100% sure if this is the "right" fix, the alternative is to change the value that is passed to FCurve->array_index, but it I'm also not sure how this is passed.