Don't crash ksmserver if restoreSession is called twice

The connection to the lambda persists, so when it's called a second time, the lambda is also called twice, eventually resulting in a nullptr deref as m_restoreSessionCall is empty.

Originally reported downstream at http://bugzilla.opensuse.org/show_bug.cgi?id=1176572, where calling "vncserver" inside a Plasma session started a nested one without its own DBus. That has to be fixed, but that this crashes the existing session has to be avoided.