Bug 20126: (follow-up bug 10455) Restore MARC field length calculation in ModBiblioMarc

Bug 20126: (follow-up bug 10455) Restore MARC field length calculation in ModBiblioMarc

Trivial fix. Problem raised by bug 10455.

Test plan:
[1] Create or edit biblio record.
[2] Save and check leader field lengths in MARC view.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
(cherry picked from commit ef7f119b1e0c0f5403239213e9ff0f450c923acf)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>