Fix regressions related to stacking order changes

The change to have stacking order containing
Toplevel inestead of Clients (431aad6d)
introduced possible crashers as recognized in:
http://git.reviewboard.kde.org/r/104519/#review12714

This change fixes these issues by adding missing
null pointer checks after casting to Client.

BUG: 298669
FIXED-IN: 4.9.0
REVIEW: 104714