Comment 13 for bug 879043

I've added a check for missing icons in https://github.com/PintaProject/Pinta/commit/c596de0ee62fa284fd9f39d37f12755a48386170, and we now show a "missing image" icon if a stock icon is missing for some reason.

I think this should prevent the crashes mentioned in this bug report. Can someone verify that this indeed fixes the issue?