afl: divide-by-zero

(cherry picked from commit eb6d27321d2d5f9d069c4a3cbcc9bc6e5b4c98ab)

Change-Id: Ided311873f654c0f40dae57c8876a6412ee97d3e
Reviewed-on: https://gerrit.libreoffice.org/12515
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>