deposit: fix dynamic field list addition

* Fixes case in `update_element` function when the field index increases.
  (closes #1784) (PR #2372)

Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Signed-off-by: Konstantinos Kostis <konstantinos.kostis@cern.ch>
Tested-by: Georgios Kokosioulis <giokokos@gmail.com>