Log Message:
po4a: update to 0.57.

=======================================================================
        ___   ____ _____
__   __/ _ \ | ___|___  |
\ \ / / | | ||___ \  / /       The Diwali release.
 \ V /| |_| | ___) |/ /
  \_/  \___(_)____//_/         (released 2019-10-25)

AsciiDoc:
 * Fix a crash occuring when a cell is nothing.

Translations:
 * New translation: Hungarian, thanks Balázs Úr.
 * Updated: French, thanks brandelune, Mattéo Rossillol‑‑Laruelle and
   Julien Lepiller.
 * Updated: German, thanks Helge Kreutzmann.
 * Updated: Italian, thanks Marco Ciampa.
 * Updated: Norwegian Bokmål, thanks Allan Nordhøy.
 * Updated: Polish, thanks Robert Luberda.
 * Updated: Portuguese, thanks Silvério Santos.
 * Updated: Portuguese (Brazil), thanks Rafael Fontenelle.
 * Updated: Spanish, thanks Adolfo Jayme Barrientos.
 * Updated: Swedish, thanks Anders Jonsson.
 * Updated: Ukrainian, thanks Yuri Chornoivan (українська).