prevents division by 0 in updater

Hi,
on my mothers Windows 7 PC, the updater crashes at 5%.
This should fix it:

```C++
+ if (diff.size() == 0){
+ fatalError(tr("The diff list is empty."));
+ }
setProgress(5);
/// 2. Check the update (5-50%)
float checkProgressStep = 45/diff.size();
```