Please describe the problem:
When leaving message via gnome-screensaver the clipboard buffer persist closure
of the "Leave message" dialog, but only when the procedure of posting the
message is interrupted via "Cancel" button. The "Save" button does the mock up.

Steps to reproduce:
1. Lock the screen, GNOME Screensaver will pop up
2. Hit "Leave message"
3. Write something, select it, copy to clipboard via Control+C
4. Hit "Cancel"
5. Hit "Leave message"
6. Press Control+P

Actual results:
The buffer from previous editing is pasted.

Expected results:
Buffer is cleared on "Cancel" hit.

Does this happen every time?
Yes, every time.

Other information:
The user who did Control+C could be potentially different from the one who
pressed Control+P and (parts of) the information sent to the "locked" user
might be revealed.

After hitting "Cancel" in the "unlock" screen itself, not in the "Leave
message" dialog, the clipboard buffer is cleared OK.