Desktop sharing doesn't work when there's no camera attached

[zetok]

Brief Description

Desktop sharing doesn't work when there's no camera attached.

OS: Linux
Commit hash: a01293f
Qt: 5.6.1

Reproducible: Always

Steps to reproduce

1. Start qTox on system that has no camera attached, and didn't have one attached
2. Go to settings in order to try to use Desktop as video device

Observed Behavior

Even though device seems to be selected, there's no screen in screen preview, and logs don't show info about screen size.

Expected Behavior

Video preview should be shown

Additional Info

Note: I've tried to reproduce that after detaching camera without restarting OS, but I wasn't able to.

[abbat]

Steps to reproduce:

• Attach camera (or run qTox with attached camera);
• Switch to camera in preview tab;
• Change tab;
• Detach camera;
• Return to settings tab and try to switch to Desktop camera;

[BiTOk]

I take it.