workspace: fix crash when creating a new workspace with sticky windows that have struts

The constructor would collect windows that are sticky before initializing its state
which would lead to a crash in the case of windows with struts which trigger a work
area recalculation where mutter would assume, due to uninitialized state, that an
existing work area has to be freed.

https://bugzilla.gnome.org/show_bug.cgi?id=738384