This bug occured when a second telepathy tubes client was connected after
the first one had disconnected and the channel (thus, the screen too)
had been destroyed.

git-svn-id: svn://anonsvn.kde.org/home/kde/trunk/KDE/kdenetwork/krfb/libvncserver@1195288 283d02a7-25f6-0310-bc7c-ecb5cbfe19da