Fix crash when creating a new box
When choosing the option to create a new box, gnome-boxes crashes. In the terminal, a lot of
'Failed to find OS with id:...' messages are shown.
After analyzing it, I found that the problem is that it tries to fill three recommended downloads, but only two are available; all the others failed to be found. Thus, when trying to fill the third, it crashes due to a NULL reference.
This patch fixes this by ensuring that it will never try to show more than as many recommended OS are in the list. It also manages the case when all the list download fails and
get_recommended_downloads() returns NULL.
Fix #504 (closed)