[Libreoffice] [REVIEW] fix for a chart crasher

[Tor Lillqvist]

> for a cherry-pick candidate to libreoffice-3-3.  It's a very simple &
> safe fix, and fixes a crasher as reported in

But do you know how those values can end up being negative in the first place, is that a symptom of something being horribly wrong otherwise, and this patch now then just plasters over that and instead of crashing it might permit silent data corruption elsewhere then instead?

--tml