Fixed Clipboard Reference Image Bug and Safe-Assert

It was possible to assign the Link to Image combobox option to
references with no local file. Krita wouldn't crash but fail gracefully
with a Safe-Assert. I added more preventative measures to make sure that
the assert isn't called and made it clear to the users that it is not
possible to link that reference to a local file (combobox option is grayed out
and impossible to select.)