[REVIEW-3-6, 3-6-1] fix crash in cond format export code with a "broken" document, fdo#53339

[Markus Mohrhard]

Hey,

I'd like [1] to be backported to 3-6 and 3-6-1. It fixes a crash that
should only happen with this insane document but it is a safe fix and
can't harm in normal cases.

Please just merge it with [2] before pushing.

Regards,
Markus

[1] http://cgit.freedesktop.org/libreoffice/core/commit/?id=631b6e87ca95c0e2bcb2bdc34095c5f601af3469
[2] http://cgit.freedesktop.org/libreoffice/core/commit/?id=42c35f0d5bea1eafdbc02b92febaed1dcb2ab332