deposit: fix dynamic field list addition
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 <2bdffcb416fed3002465740d19a053a39918cbe5@cern.ch>
Signed-off-by: Konstantinos Kostis <237b836d1007d10b89d97a8c1749c99bbd9efc26@cern.ch>
Tested-by: Georgios Kokosioulis <b53d18f1114803d6aa1ea365a38c33279721adb3@gmail.com>

inveniosoftware/invenio-deposit,inveniosoftware/invenio-deposit,inveniosoftware/invenio-deposit,inveniosoftware/invenio-deposit