valgrind + bff: Integer divide by zero

Change-Id: I4c47d17a421f1c6d2d6cc29c7b92ec930bdf901f
(cherry picked from commit a1fab361cb3325b2d012942e43660f2c0a2a4b96)
Reviewed-on: https://gerrit.libreoffice.org/11021
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>