Fixed one of biggest Okular crashers

I just fixed one of the biggest Okular crashers https://bugs.kde.org/show_bug.cgi?id=243434 (around 20 to 30 duplicate reports). This one was not really difficult to fix (actually it is just a one liner), but it was difficult to fix for me since I could not reproduce it

It took a while to find someone that had the crash and was willing to help me debug it (thanks Adrià). So please, all users reading this blog, be more proactive when we ask questions in bugs, because, you know, it is in your best interest answering our questions and helping us pinpoint where the problem is.