Overwriting dst_img->type[0] inside babl-fish-reference.c
dst_img->type[0] = (BablType *) babl_type_from_id (BABL_DOUBLE);
dst_img->pitch[0] = destination_fmt->type[0]->bits/8;
dst_img->stride[0] = 0;
dst_img->type[0] = destination_fmt->type[0];
https://gitlab.gnome.org/GNOME/babl/-/blob/master/babl/babl-fish-reference.c#L361-365
and also
https://gitlab.gnome.org/GNOME/babl/-/blob/master/babl/babl-fish-reference.c#L483-487